Add picker for opening a workspace in one or more directories

[joe-p]

As a user, I want to be able to switch to a different workspace even if I haven't opened it recently. As someone that previously used wezterm for switching projects , I found the flow for opening projects to be a friction point when switching from wezterm+neovim to zed.

The functionality would basically be the recent projects picker but the ability to define specific sub-directories. For example:

      "cmd-k": [
        "projects::OpenSubdirectories",
        {
          "base_directories": [
            "/Users/joe/git/joe-p",
            "/Users/joe/git/other"
          ],
          "create_new_window": true
        }
      ]

Will let me pick from all the subdirs under joe-p and other.

I have this spike out here: https://github.com/zed-industries/zed/compare/main...joe-p:feat/subdir_picker?expand=1

Most of the legwork was done by Sonnet 4.5