Add recipe to add XML declaration to Jelly files

Fixes #1

Implement the `AddJellyXmlDeclaration` recipe to add the XML declaration to Jelly files.

* **AddJellyXmlDeclaration.java**
  - Implement the `AddJellyXmlDeclaration` class to add the XML declaration to Jelly files.
  - Use `PlainTextVisitor` to modify the content of `.jelly` files.
  - Ensure the XML declaration `<?jelly escape-by-default='true'?>` is present in all `.jelly` files.

* **AddJellyXmlDeclarationTest.java**
  - Implement the `AddJellyXmlDeclarationTest` class to test the `AddJellyXmlDeclaration` recipe.
  - Write tests to verify the XML declaration is added to Jelly files.
  - Ensure tests cover cases where the declaration is already present and where it is missing.

---

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src/main/java/org/openrewrite/jenkins/AddJellyXmlDeclaration.java

@@ -0,0 +1,75 @@
+/*
+ * Copyright 2023 the original author or authors.
+ * <p>
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * <p>
+ * https://www.apache.org/licenses/LICENSE-2.0
+ * <p>
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.openrewrite.jenkins;
+
+import org.openrewrite.ExecutionContext;
+import org.openrewrite.Recipe;
+import org.openrewrite.text.PlainText;
+import org.openrewrite.text.PlainTextVisitor;
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+
+/**
+ * Recipe to add an XML declaration to Jelly files.
+ */
+public class AddJellyXmlDeclaration extends Recipe {
+
+    private static final Logger LOG = LoggerFactory.getLogger(AddJellyXmlDeclaration.class);
+
+    @Override
+    public String getDisplayName() {
+        return "Add XML declaration to Jelly files";
+    }
+
+    @Override
+    public String getDescription() {
+        return "Ensure the XML declaration `<?jelly escape-by-default='true'?>` is present in all `.jelly` files.";
+    }
+
+    @Override
+    public PlainTextVisitor<ExecutionContext> getVisitor() {
+        return new PlainTextVisitor<ExecutionContext>() {
+            public static final String JELLY_DECLARATION = "<?jelly escape-by-default='true'?>";
+
+            @Override
+            public PlainText visitText(PlainText text, ExecutionContext executionContext) {
+                if (text == null || text.getSourcePath() == null) {
+                    return text;
+                }
+                if (text.getSourcePath().toString().endsWith(".jelly")) {
+                    LOG.debug("Processing Jelly file: {}", text.getSourcePath());
+                    String content = text.getText();
+                    if (content.trim().isEmpty()) {
+                        LOG.debug("Adding declaration to empty file");
+                        return text.withText(JELLY_DECLARATION);
+                    }
+                    String lineEnding = content.contains("\r\n") ? "\r\n" : "\n";
+                    if (content.trim().toLowerCase().matches("^<\\?jelly\\s+[^>]*>") && !content.startsWith(JELLY_DECLARATION)) {
+                        LOG.warn("Found malformed Jelly declaration in {}", text.getSourcePath());
+                        LOG.debug("Adding missing declaration");
+                        content = content.substring(content.indexOf(lineEnding) + lineEnding.length());
+                    }
+                    if (!content.startsWith(JELLY_DECLARATION)) {
+                        LOG.debug("Declaration already present");
+                        content = JELLY_DECLARATION + lineEnding + content;
+                        return text.withText(content);
+                    }
+                }
+                return text;
+            }
+        };
+    }
+}

src/test/java/org/openrewrite/jenkins/AddJellyXmlDeclarationTest.java

@@ -0,0 +1,69 @@
+package org.openrewrite.jelly;
+
+import static org.openrewrite.test.SourceSpecs.text;
+
+import java.io.IOException;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.io.TempDir;
+import org.openrewrite.Recipe;
+import org.openrewrite.test.RewriteTest;
+import org.openrewrite.text.PlainTextParser;
+
+/**
+ * Test class for the AddJellyXmlDeclaration recipe.
+ */
+public class AddJellyXmlDeclarationTest implements RewriteTest {
+
+    @Override
+    public Recipe getRecipe() {
+        return new AddJellyXmlDeclaration();
+    }
+
+    @Test
+    void addXmlDeclarationToJellyFile(@TempDir Path tempDir) throws IOException {
+        Path inputFile = tempDir.resolve("example.jelly");
+        Files.writeString(inputFile, """
+            <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define">
+                <st:contentType value="text/html"/>
+                <h1>Hello, World!</h1>
+            </j:jelly>
+        """);
+
+        Path expectedFile = tempDir.resolve("expected.jelly");
+        Files.writeString(expectedFile, """
+            <?jelly escape-by-default='true'?>
+            <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define">
+                <st:contentType value="text/html"/>
+                <h1>Hello, World!</h1>
+            </j:jelly>
+        """);
+
+        rewriteRun(
+                spec -> spec.recipe(new AddJellyXmlDeclaration())
+                        .parser(PlainTextParser.builder())
+                        .expectedCyclesThatMakeChanges(1)
+                        .cycles(1),
+                text(Files.readString(inputFile), Files.readString(expectedFile)));
+    }
+
+    @Test
+    void doNotAddXmlDeclarationIfAlreadyPresent(@TempDir Path tempDir) throws IOException {
+        Path inputFile = tempDir.resolve("example.jelly");
+        Files.writeString(inputFile, """
+            <?jelly escape-by-default='true'?>
+            <j:jelly xmlns:j="jelly:core" xmlns:st="jelly:stapler" xmlns:d="jelly:define">
+                <st:contentType value="text/html"/>
+                <h1>Hello, World!</h1>
+            </j:jelly>
+        """);
+
+        rewriteRun(
+                spec -> spec.recipe(new AddJellyXmlDeclaration())
+                        .parser(PlainTextParser.builder())
+                        .expectedCyclesThatMakeChanges(1)
+                        .cycles(0),
+                text(Files.readString(inputFile)));
+    }
+}