Prompt to update PowerShell version

[TylerLeonhardt]

PR Summary

This adds a little dialog that informs you that you can update your version of PowerShell.

It will update:

6.0 - 6.2.1 -> 6.2.2
any preview version -> 7.0.0-preview.2

here's an example:

It updates via homebrew on macOS and through the MSI (interactively) on Windows.

Linux is not supported.

This won't "update" Windows PowerShell to PowerShell 6+ yet as it's a trickier situation.

PR Checklist

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[bergmeister]

Good idea and relatively simple implementation (+1) for such a nice functionality. I think it needs some tweaks but otherwise looks good, don't be put off by the amount of comments.

[bergmeister]

PowerShellGitHubRPrereleasesUrl --> PowerShellGitHubPrereleasesUrl ?

[GHRoss]

I really like this.

My only concern (running macOS) is that the user might not have Brew installed, is there a check in here for that?

A quick Google returned this, which might help? https://stackoverflow.com/questions/21577968/how-to-tell-if-homebrew-is-installed-on-mac-os-x

[TylerLeonhardt]

@GHRoss Good suggestion! Since we're in typescript though, we'd want to use a node.js way of finding out if we have brew which sadly would require yet another package to be brought in.

Considering we'd have to do this, or something less than ideal like execute which ourselves and parse the response, and since PowerShell's recommended install instructions say to use Homebrew, I think we can close to assume it's available...

That said, as a compromise, I'll have the dialog explicitly say it'll use brew:

[TylerLeonhardt]

p.s. @rjmholt ...

better "never" text?

[TylerLeonhardt]

I think I've addressed all feedback here

[TylerLeonhardt]

@bergmeister I've addressed all your feedback - I'm going to merge this in today or tomorrow for a release unless you have any more feedback

[TylerLeonhardt]

I will try to backport this to legacy but if it doesn't go well I'm ok keeping this in Preview