Add workspace file to load both vscode-powershell and PSES

[rkeithhill]

PR Summary

It is convenient to have both the extension and PSES loaded at the same time into VSCode. I would think that anyone working on the extension would want that. The name I have for it currently is extension-dev.code-workspace but I'm open to whatever name folks prefer. If we don't want this then I'd like to instead add an entry to .gitignore to ignore *.code-workspace files. That said, I don't see much harm in adding the workspace file. Folks don't have to use it.

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[TylerLeonhardt]

LGTM I was thinking about this recently too