This is a step towards making it Send+Sync.
I'm a little cagey about this in the long term, as there are some mux
operations that may technically require multiple fields to be locked for
their duration: allowing free-threaded access may introduce some subtle
(or not so subtle!) interleaving conditions where the overall mux state
is not yet consistent.
I'm thinking of prune_dead_windows kicking in while the mux is in the
middle of being manipulated.
I did try an initial pass of just moving everything under one lock, but
there is already quite a lot of mixed read/write access to different
aspects of the mux.
We'll see what bubbles up later!
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
