This PR adds es2015 lib to the `tsconfig.json` of the test project so
that `Promise` could be used. Note this only affects diagnostics in the
IDE. The tsconfig in Language Service Mock Host is the actual config
values used, and it already has es2015 lib.
- Other minor cleanup: Rename imports in `main.ts`.
- Add more cases to `parsing-cases.ts`, which are tested in later PRs
PR Close#33157
This is the last part in refactoring of the test project.
This PR turns on strict mode for typechecking and fixed tests that
fail under this mode.
PR Close#32783