fix(language-service): incorrect autocomplete results on unknown symbol (#37518)

This commit fixes a bug whereby the language service would incorrectly
return HTML elements if autocomplete is requested for an unknown symbol.
This is because we walk through every possible scenario, and fallback to
element autocomplete if none of the scenarios match.

The fix here is to return results from interpolation if we know for sure
we are in a bound text. This means we will now return an empty results if
there is no suggestions.

This commit also refactors the code a little to make it easier to
understand.

PR Close #37518

packages/language-service/test/completions_spec.ts

@@ -841,6 +841,13 @@ describe('completions', () => {
       'trim',
     ]);
   });
+
+  it('should not return any results for unknown symbol', () => {
+    mockHost.override(TEST_TEMPLATE, '{{ doesnotexist.~{cursor} }}');
+    const marker = mockHost.getLocationMarkerFor(TEST_TEMPLATE, 'cursor');
+    const completions = ngLS.getCompletionsAtPosition(TEST_TEMPLATE, marker.start);
+    expect(completions).toBeUndefined();
+  });
 });
 
 function expectContain(