Return `TemplateDiagnostic` instead of `ts.Diagnostic` when getting the
extended template diagnostics. This makes the integration with the
language service easier. This also fixes the error code and now uses the
`ngErrorCode` for extended template diagnostics.
Refs #42966
PR Close#43134
This commit adds extended template diagnostics end-to-end tests, to make
sure the diagnostics are generated correctly. Template checks are
already tested with unit tests.
Refs #42966
PR Close#43107