Handle name better

[TylerLeonhardt]

PR Summary

Fixes #1721

For some reason, vscode has the storagePath set to null on untitled workspaces... this uses a different path as a fallback if that's not available. storagePath is the most accurate when it's set. extensionPath isn't fully accurate when running a local build of the extension because the extensionPath ends up being (on my machine):

~/Code/PowerShell/vscode/vscode-powershell

That said... hopefully I'll be able to address the initial limitation: microsoft/vscode#66515

Also, I added some tests and fixed up enabling the tests on non-windows.

PR Checklist

Note: Tick the boxes below that apply to this pull request by putting an x between the square brackets.
Please mark anything not applicable to this PR NA.

• 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

[TylerLeonhardt]

Looks like the test runner doesn't work on Linux... but it works on macOS so I kept macOS enabled.