Implement custom workspace folder picker

[isc-bsaviano]

• More info than built-in showWorkspaceFolderPick().
• Allows for filtering invalid folders before prompting the user.
• Will not prompt the user if there is only one folder to pick from.
• Differentiates between user cancel and nothing to pick from.

[gjsjohnmurray]

Code looks good, but I haven't tested it.

Now that we are in full control of the quickpick we could add a Close button to the titlebar to give a mouse-only way of dismissing it, since we're retaining the use of ignoreFocusOut: true which means clicking outside it doesn't dismiss it. See microsoft/vscode#99081 for an open issue proposing doing this automatically some day.

However I don't feel strongly that we should do the extra work in this PR, so if you'd prefer not to then let me know and I'll approve it as it stands.

[isc-bsaviano]

Thanks for reviewing and linking that issue. I was planning on doing a subsequent "UI normalization" PR that normalizes some language and the use of title instead of placeHolder so I can add this for some of our QuickPicks as part of that PR. I am also ok with removing ignoreFocusOut if we think that's inaccessible.