Implement "open folder" command

[lykius]

Hi @rlivings39!
Very nice extension!

Could it be possible to implement the command "File: open folder..."?
I know that the extension already support "fzf: Add workspace folder using fzf...", but this is slightly different from simply opening the folder.
I'm no expert in TS and VSCode extension, but if you don't have time for that I will try to create a PR for this.

Thanks in advance!
Luca