add FileLinePositionSpanConverter (#278)

src/Tests/Tests.FileLinePositionSpanConverter.verified.txt

@@ -0,0 +1 @@
+the path: (1,2)-(2,4)

src/Tests/Tests.cs

@@ -17,6 +17,10 @@ public Task SourceTextConverter() =>
     public Task LocalizableStringConverter() =>
         Verify(new LocalizableStringImp());
 
+    [Fact]
+    public Task FileLinePositionSpanConverter() =>
+        Verify(new FileLinePositionSpan("the path", new(1,2), new(2,4)));
+
     class LocalizableStringImp :
         LocalizableString
     {

src/Verify.SourceGenerators/Converters/FileLinePositionSpanConverter.cs

@@ -0,0 +1,8 @@
+using Microsoft.CodeAnalysis;
+
+class FileLinePositionSpanConverter :
+    WriteOnlyJsonConverter<FileLinePositionSpan>
+{
+    public override void Write(VerifyJsonWriter writer, FileLinePositionSpan value) =>
+        writer.WriteValue(value.ToString());
+}

src/Verify.SourceGenerators/Converters/LocationConverter.cs

@@ -4,5 +4,5 @@ class LocationConverter :
     WriteOnlyJsonConverter<Location>
 {
     public override void Write(VerifyJsonWriter writer, Location value) =>
-        writer.WriteValue(value.GetMappedLineSpan().ToString());
+        writer.WriteValue(value.GetMappedLineSpan());
 }

src/Verify.SourceGenerators/VerifySourceGenerators.cs

@@ -22,6 +22,7 @@ public static void Initialize()
             converters.Add(new LocalizableStringConverter());
             converters.Add(new DiagnosticConverter());
             converters.Add(new LocationConverter());
+            converters.Add(new FileLinePositionSpanConverter());
             converters.Add(new GeneratedSourceResultConverter());
             converters.Add(new DiagnosticDescriptorConverter());
             converters.Add(new SourceTextConverter());