The Language Service "find references" currently uses the
`ngtypecheck.ts` suffix to determine if a file is a shim file. Instead,
a better API would be to expose a method in the template type checker
that does this verification so that the LS does not have to "know" about
the typecheck suffix. This also fixes an issue (albeit unlikely) whereby a file
in the user's program that _actually_ is named with the `ngtypecheck.ts`
suffix would have been interpreted as a shim file.
PR Close#39768