This commit refactors the previously introduced `getGlobalCompletions()` API
for the template type-checker in a couple ways:
* The return type is adjusted to use a `Map` instead of an array, and
separate out the component context completion position. This allows for a
cleaner integration in the language service.
* A new `CompletionEngine` class is introduced which powers autocompletion
for a single component, and can cache completion results.
* The `CompletionEngine` for each component is itself cached on the
`TemplateTypeCheckerImpl` and is invalidated when the component template
is overridden or reset.
This refactoring simplifies the `TemplateTypeCheckerImpl` class by
extracting the autocompletion logic, enables caching for better performance,
and prepares for the introduction of other autocompletion APIs.
PR Close#39278
c4f99b6e52