Check if the file added is emitted file after validating extensions

[sheetalkamat]

May fix #21274

[sheetalkamat]

@mjbvz @Tyriar could you please try out this branch to see if it helps with #21274

[mjbvz]

@Tyriar I'm actually not seeing the perf issue on my Mac. Could you please test out this branch:

1. Check out this branch from the ts repo
2. In the folder, run npm install and then gulp local to build a local ts version
3. In VS Code, set add a user setting pointing to the tsdk build location, for example:"typescript.tsdk": "/Users/matb/projects/TypeScript/built/local"
4. Restart vscode and make sure you are using the built-in version of TS

We can also take a look at this together tomorrow

[Tyriar]

I just set up the local build, will report back soon.

[Tyriar]

Nothing happened so far, so I guess it's fixed. The repro I had before happened about 5 minutes into opening VS Code, this happened 3 times in a row fairly consistently. Will ping back if I see it but I think this fixes the problem.

[sheetalkamat]

@mhegazy can you please review this change, so we can take this in.