refactor(language-service): Remove NgLSHost -> NgLS dependency (#31122)

```
NgLSHost: AngularLanguageServiceHost
NgLS: AngularLanguageService
```

NgLSHost should not depend on NgLS, because it introduces circular
dependency.
Instead, the `getTemplateAst` and `getTemplatAstAtPosition` methods should
be moved to NgLSHost and exposed as public methods.
This removes the circular dependency, and also removes the need for the
awkward 'setSite' method in NgLSHost.

PR Close #31122

packages/language-service/test/completions_spec.ts

@@ -22,7 +22,6 @@ describe('completions', () => {
   let service = ts.createLanguageService(mockHost, documentRegistry);
   let ngHost = new TypeScriptServiceHost(mockHost, service);
   let ngService = createLanguageService(ngHost);
-  ngHost.setSite(ngService);
 
   it('should be able to get entity completions',
      () => { contains('/app/test.ng', 'entity-amp', '&', '>', '<', 'ι'); });