Surface environment variable information in hover, even when not stale

[Tyriar]

Currently when a terminal environment is "stale" we show messages here:

And here:

We have wanted to remove the latter as part of #151160.

Something I've always wanted since this was added was to surface existing environment variable contributions here as well and the upcoming description API (#171108) will make this a lot nicer. For example, the git extension could explain that it's contributing an authentication provider instead of a bunch of mysterious environment variables.

[Tyriar]

cc @karrtikr