tsc -watch should ignore files without extensions

Steps to reproduce:

1. npm ci

2. npm run watch

3. touch foo

4. Observe the incorrect output by tsc:

   File change detected. Starting incremental compilation...

5. touch index.ts

6. Observe the correct output by tsc:

   File change detected. Starting incremental compilation...

   Found 0 errors. Watching for file changes.

7. touch bar.txt

8. Observe that tsc doesn't output anything and ignores the file.