Added Symbol Kinds for Dart

[matthewnitschke-wk]

Adds support for 2 SymbolKind types that will be utilized by scip-dart

• Extension is for annotating dart's extension types, specifically the extension declaration, used for resolving api conflicts

  extension Foo on String {
  //        ^^^ Kind.Extension
    void foo() => print('foo');
  }

  Open to suggestions on a different kind to use for this use case, or even a better name if Extension is to generalized

• Mixin is for annotating dart mixins

  mixin Foo {
  //    ^^^ Kind.Mixin
      int someField;
  }

Test plan

[varungandhi-src]

It's OK to add language-specific cases to Kind. 👍🏽