Improve Settings Editor experience in a few places

[TylerLeonhardt]

PR Summary

This PR is two commits:

• two low hanging fruits that allow the CommandExplorerExcludeFilter and featureFlags settings to show up in the settings editor:
  

• a reworking of the powerShellAdditionalExePaths to be an object of { [versionName: string]: "exePath" } so that the user has a nicer experience in the settings editor. It's not perfect (I wish the Item and Value headers could be renamed), but it's the best we can do with the settings API today and think it's worth it.
  

This also lights up nice things like this:

jumping to this:

PR Checklist

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• PR has a meaningful title
• Summarized changes
• PR has tests
• This PR is ready to merge and is not work in progress
  • If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready

[andyleejordan]

Love it!!!

[TylerLeonhardt]

@andschwa 🚢