enh(scala) add Scala 3 extension soft keyword

Based on https://github.com/scala/vscode-scala-syntax/blob/57e0829cd46980699570101a68c320a20330d36c/src/typescript/Scala.tmLanguage.ts#L1134.
Also see https://docs.scala-lang.org/scala3/reference/contextual/extension-methods.html#syntax.

CHANGES.md

@@ -16,6 +16,7 @@ Grammars:
 - enh(css/less/stylus/scss) add support for CSS Grid properties [monochromer][]
 - enh(java) add support for Java Text Block (#3322) [Teletha][]
 - enh(scala) add missing `do` and `then` keyword (#3323) [Nicolas Stucki][]
+- enh(scala) add Scala 3 `extension` soft keyword (#3326) [Nicolas Stucki][]
 
 [Austin Schick]: https://github.com/austin-schick
 [Josh Goebel]: https://github.com/joshgoebel

src/languages/scala.js

@@ -117,6 +117,11 @@ export default function(hljs) {
     contains: [ NAME ]
   };
 
+  const EXTENSION = {
+    className: 'keyword',
+    begin: /(?<=^\s*)extension(?=\s+[\[\(])/
+  };
+
   return {
     name: 'Scala',
     keywords: {
@@ -132,6 +137,7 @@ export default function(hljs) {
       METHOD,
       CLASS,
       hljs.C_NUMBER_MODE,
+      EXTENSION,
       ANNOTATION
     ]
   };

test/markup/scala/extension.expect.txt

@@ -0,0 +1,19 @@
+<span class="hljs-keyword">extension</span> (x: <span class="hljs-type">Int</span>) <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">plus</span></span>(y: <span class="hljs-type">Int</span>) = x + y
+
+<span class="hljs-keyword">extension</span> [<span class="hljs-type">T</span>](x: <span class="hljs-type">T</span>) <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span> </span>= ()
+
+<span class="hljs-keyword">extension</span> (x: <span class="hljs-type">Int</span>)
+  <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span> </span>= <span class="hljs-number">1</span>
+  <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span> </span>= <span class="hljs-number">2</span>
+
+<span class="hljs-class"><span class="hljs-keyword">object</span> <span class="hljs-title">Foo</span> </span>{
+  <span class="hljs-keyword">extension</span> (x: <span class="hljs-type">Int</span>)
+    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span> </span>= <span class="hljs-number">1</span>
+    <span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">f</span> </span>= <span class="hljs-number">2</span>
+}
+
+<span class="hljs-function"><span class="hljs-keyword">def</span> <span class="hljs-title">extension</span></span>(file: <span class="hljs-type">File</span>) =
+  file.extension
+
+  file
+    .extension

test/markup/scala/extension.txt

@@ -0,0 +1,19 @@
+extension (x: Int) def plus(y: Int) = x + y
+
+extension [T](x: T) def f = ()
+
+extension (x: Int)
+  def f = 1
+  def f = 2
+
+object Foo {
+  extension (x: Int)
+    def f = 1
+    def f = 2
+}
+
+def extension(file: File) =
+  file.extension
+
+  file
+    .extension