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/symbols.ts

@@ -129,6 +129,11 @@ export interface Symbol {
    * If the symbol cannot be indexed, this method should return `undefined`.
    */
   indexed(argument: Symbol, key?: any): Symbol|undefined;
+
+  /**
+   * Returns the type arguments of a Symbol, if any.
+   */
+  typeArguments(): Symbol[]|undefined;
 }
 
 /**