refactor(language-service): adds Symbol#typeArguments and does cleanup (#34571)

Adds a `typeArguments` method to the `Symbol` interface, cleaning up how
type parameters of a TypeScript type are currently found. This will be
necessary for providing completions for `$event` variables' properties
(#34570).

This commit also performs some fly-by cleanups seen while implementing
the `typeArguments` methods. There is more clean up to do in the
`typescript_symbols` file, but the scope of this commit didn't need to
get larger.

PR Close #34571

packages/language-service/src/locate_symbol.ts

@@ -223,4 +223,6 @@ class OverrideKindSymbol implements Symbol {
   selectSignature(types: Symbol[]) { return this.sym.selectSignature(types); }
 
   indexed(argument: Symbol) { return this.sym.indexed(argument); }
+
+  typeArguments(): Symbol[]|undefined { return this.sym.typeArguments(); }
 }