This commit makes the Angular Language Service interface a strict subset
of TypeScript's Language Service by renaming all methods to be
consistent with TypeScript's.
The custom Angular `LanguageService` interface was needed before the
inception of TypeScript tsserver plugin, but is now obsolete since
Angular LS is a proper tsserver plugin.
This allows us to easily adapt to upstream TS changes in the future, and
also allows us to reuse all data types defined in TypeScript.
PR Close#34888