Support adding more than one directory to a workspace

[aloussase]

How I imagined this plugin would work:

WorkspacesAdd w1 dir1
WorkspacesAdd w1 dir2

Then, opening nvim in either directory would activate the workspace.

In terms of implementation, this could work by creating a workspace directory with symlinks to the directory workspaces.

Then, when opening either registered path, we'd just chdir into the directory with the symlinks.