refactor(language-service): Differentiate Inline and External template (#32127)

This commit creates two concrete classes Inline and External
TemplateSource to differentiate between templates in TS file and
HTML file.
Knowing the template type makes the code much more explicit which
filetype we are dealing with.

With these two classes, there is no need for `getTemplateAt()` method in
TypeScriptHost. Removing this method is safe since it is not used in the
extension. This reduces the API surface of TypescriptHost.

PR Close #32127

packages/language-service/src/language_service.ts

@@ -36,7 +36,7 @@ class LanguageServiceImpl implements LanguageService {
     const results: Diagnostic[] = [];
     const templates = this.host.getTemplates(fileName);
     for (const template of templates) {
-      const ast = this.host.getTemplateAst(template, fileName);
+      const ast = this.host.getTemplateAst(template);
       results.push(...getTemplateDiagnostics(template, ast));
     }
     const declarations = this.host.getDeclarations(fileName);