Add isProposedApiEnabled check for env var scope

[Tyriar]

It's used in #182883, this would be easiest after that's merged.

[karrtikr]

https://github.com/microsoft/vscode/pull/182883/files#r1202854515

[karrtikr]

I wish instead of explicitly checking whether proposed API is enabled in package.json in the implementation, we could check it when checking distro for whether an extension is allowed a proposal instead.

[Tyriar]

@karrtikr the list of allowed extensions is not available in code - oss though on local or in CI

[karrtikr]

That's fair but do we really need to check in code - oss when developing? I would think distro is being tested out in the final build that's published.

[Tyriar]

@karrtikr not my area but being strict is good for proposals and it's good to align oss with the product in areas like this. Consider a fork of VS Code that does not have the check, extensions could use the proposals without warning which could lead to easy breakages.