This PR fixes #98446.

It introduces a new flag in the IMonarchLanguage interface, giving developers the ability to use unicode-aware regular expressions in their Monaco Monarch language specifications.

A separate PR is needed to update Monaco Monarch's documentation:
https://github.com/microsoft/monaco-editor/blob/master/website/monarch.html

It's also important to let developers know that the unicode flag may break existing regular expressions (i.e., /\"/ works, but /\"/u fails).

For more details, see: https://mathiasbynens.be/notes/es6-unicode-regex