Support extension on

```
extension on ...
extension <ident> on ...
extension { ...
extension <ident> { ...
```

src/typescript/Scala.tmLanguage.ts

@@ -864,13 +864,26 @@ export const scalaTmLanguage: TmLanguage = {
     },
     extension: {
       patterns: [
+        {
+          match: `(extension)\\s+(?:(${plainid})\\s)?\\s*(on|(?=\\{))`,
+          captures: {
+            '1': {
+              name: 'keyword.declaration.scala'
+            },
+            '2': {
+              name: 'entity.name.class'
+            },
+            '3': {
+              name: 'keyword.declaration.scala'
+            }
+          }
+        },
         {
           match: `(extension)\\s+(?=[\\[\\(])`,
           captures: {
             '1': {
               name: 'keyword.declaration.scala'
             },
-
           }
         }
       ]

tests/unit/extension-on.test.scala

@@ -0,0 +1,23 @@
+// SYNTAX TEST "source.scala"
+
+    extension on (x: Rational):
+//  ^^^^^^^^^ keyword.declaration.scala
+//            ^^ keyword.declaration.scala
+//                ^ variable.parameter.scala
+//                   ^^^^^^^^ entity.name.class
+       def > (y: Rational): Boolean = y < x
+
+   extension Ops on (x: Rational):
+// ^^^^^^^^^ keyword.declaration.scala
+//           ^^^ entity.name.class
+//               ^^ keyword.declaration.scala
+//                   ^ variable.parameter.scala
+//                      ^^^^^^^^ entity.name.class
+      def > (y: Rational): Boolean = y < x
+
+   extension stringOps {  }
+// ^^^^^^^^^ keyword.declaration.scala
+//           ^^^^^^^^^ entity.name.class
+
+   extension {  }
+// ^^^^^^^^^ keyword.declaration.scala