The intent is to expose Mux related functions to lua, so `wezterm.mux`
will be that module table.
In order to test this out in the debug overlay, I realized that the
overlay was running functions in a different thread and didn't have
access to the mux, so this commit also tweaks the debug overlay repl to
execute the input in the main thread.
The result is that it is now possible to do
`wezterm.mux.active_workspace()` in the debug overlay to print the
active workspace name.
More functions will follow.
refs: https://github.com/wez/wezterm/issues/225
Main thing to note here is that the open crate has deprecated
open::that_in_background, but made open::that non-blocking.
I think this is OK, but I'm a little cagey about what will
happen with this on Windows. We may need to spawn our own
thread for this if things go awry.
