Implement `gitspaces [setup]` - default action when no ~/.gitspaces folder

[davfive]

Background

After a user runs go install https://github.com/davfive/gitspaces/v2@latest for the initial install and run gitspaces for the first time, they are missing two key configurations:

1. The ProjectPaths list in ~/.gitspaces/config.yml
   This list is used to search for projects when switching spaces

2. The shell wrapper
   gitspaces often needs to cd into the requested space after an action (e.g., create, sleep, rename, switch). Since the go/bin/gitspaces runs as a subprocess, it can't change directories in the calling (parent) shell. To address this, I use a shell function wrapper of the same name which looks for a file after go/bin/gitspaces exits successfully and cd's to it if found.

Requirements

1. Until setup is complete and go/bin/gitspaces is being called by the shell function, provide instructions to run the . ~/.gitspaces/<shell>rc to their shell's rc file (~/.bashrc or ~/.zshrc) file.
2. Until ProjectPaths has paths in it, report an error and provide information on how to fix it.