[JS, TS] Add Monarch support for private identifiers

[spahnke]

Fixes #3254

This is a very simple solution that just allows the # character as the start character of identifiers for all identifiers. I don't know Monarch enough to know if there is a way to constrain this to identifiers in classes, but I doubt there is. In any case if you use the # character at the start of e.g. a variable name the language service raises an error diagnostic, so that this might not be a problem? Maybe this simple solution is enough for this issue. Let me know what you think, or feel free to discard this.

Local Playground Link: https://microsoft.github.io/monaco-editor/playground.html?sourceLanguages=http%3A%2F%2Flocalhost%3A5002%2Fout%2Flanguages%2Famd-tsc#XQAAAAJoAQAAAAAAAABBqQkHQ5NjdMjwa-jY7SIQ9S7DNlzs5W-mwj0fe1ZCDRFc9ws9XQE0SJE1jc2VKxhaLFIw9vEWSxW3yscwzblk-_1uhs0-3YGTBOsVJ4S4FwfCDQb5-2uUHXTVAefbVSd9IzaZ_vv9VR9KugsKikU_wkLK9jT_o8SJxwvSkZPE3r8N8C4O8JIRyP6PwIhuEzac9gRZJKV5O9_hRAHvieANI4sR5MKuH1inQaY1HbVLv0DvpOrvrDW6_vt1nHUY5cvnsy0iVQoS8hn2gWkAT4c2hWqFP24-1CdV2IAbzISiaOAa-IE1cC042gJ9VmprZ1KuCo6ic8uz7shuG3cxVt_qoOHa5r6Qz6qCXoH-gAs_coxt1vAO8lWJWMLtOTMA3__77F3Y

[hediet]

Thanks!

Also: I see you used the playground to debug/develop your PR. Did you run into any issues?

[spahnke]

Also: I see you used the playground to debug/develop your PR. Did you run into any issues?

No issues! I really like this experience and it helped me immensely with the other PR for link and editor openers I contributed to the VS Code repo, because you could see and debug the changes immediately. The only thing I had to do for my machine was to change ports, but that was easy enough.