Leverage new VS Code localization API

[karolz-ms]

VS Code is introducing a new API for localization: https://code.visualstudio.com/updates/v1_72#_localization-as-part-of-the-api

We should:

• review the localizable strings that we have in package.nls.json, leaving only those that are referenced by package.json (and thus cannot leverage the new API)
• convert all the only-used-from-code strings to use the new API

[tonybaloney]

The API isn't totally formalized, so there is a likelihood of it changing in the next release (causing backward incompatibility issues).

[karolz-ms]

@tonybaloney roger. That is why this issue is in "Backlog" milestone.