fix(ts): add incremental compilation to tsconfig.json

This fixes open-wc/open-wc#2113 - initial double reload and flicker in dev mode.

Should also make the subsequent recompiles faster also (though not measured it).

Also fixed the text in the app to say ".ts" instead of ".js".

src/generators/app-lit-element-ts/templates/MyApp.ts

@@ -53,7 +53,7 @@ export class <%= className %> extends LitElement {
         <div class="logo">${openWcLogo}</div>
         <h1>${this.title}</h1>
 
-        <p>Edit <code>src/<%= className %>.js</code> and save to reload.</p>
+        <p>Edit <code>src/<%= className %>.ts</code> and save to reload.</p>
         <a
           class="app-link"
           href="https://open-wc.org/guides/developing-components/code-examples"

src/generators/app-lit-element-ts/templates/tsconfig.json

@@ -1,4 +1,4 @@
-  {
+{
   "compilerOptions": {
     "target": "es2018",
     "module": "esnext",
@@ -13,7 +13,8 @@
     "outDir": "out-tsc",
     "sourceMap": true,
     "inlineSources": true,
-    "rootDir": "./"
+    "rootDir": "./",
+    "incremental": true
   },
   "include": ["**/*.ts"]
 }

src/generators/wc-lit-element-ts/templates/tsconfig.json

@@ -1,4 +1,4 @@
-  {
+{
   "compilerOptions": {
     "target": "es2018",
     "module": "esnext",
@@ -14,7 +14,8 @@
     "sourceMap": true,
     "inlineSources": true,
     "rootDir": "./",
-    "declaration": true
+    "declaration": true,
+    "incremental": true
   },
   "include": ["**/*.ts"]
 }