Relocate the helper function to mux-server-impl and have both the GUI
and the mux server call it at the appropriate times.
Introduce default_mux_server_domain which is used instead of
default_domain in the mux server. This is to avoid recursive
cycles when starting up the mux; we don't want the default
domain to be a unix client that connects to our selves because
we'll try to connect to ourselves, then in act of handling that
spawn in the default domain and try to connect to ourselves and
repeat.
refs: https://github.com/wez/wezterm/issues/3907
Terminal now maintains a sequence number that increments
for each Action that is applied to it.
Changes to lines are tagged with the current sequence number.
This makes it a bit easier to reason about when an individual
line has changed relative to some point in "time"; the consumer
of the terminal can sample the current sequence number and then
can later determine which lines have changed since that point
in time.
refs: https://github.com/wez/wezterm/issues/867
