Previously, stopping the read thread would not fully clean up the tty.
This could result in issues such as #9 where the terminal state was
borked upon return to shell or if temporarily exiting the TUI.
Fixes: #9
Refactor the main API to split Loop and Vaxis types. This enables the
core Vaxis type to be more easily referenced in other types, since it
doesn't require a comptime type parameter. This will make the switch to
the zg unicode library easier.
