fix for #33 and adding augroup

[mmjo]

Ubuntu 22.04 (LTS) version of vim is 8022121 which doesn't yet have the Focus* event support. Thus, the plugin is still used in 2024.

Issue 33 should be fixed with this.

It's about re-entering the window when in the command line. The message appears:

Error detected while processing function <SNR>58_do_autocmd[3]..FocusGained Autocommands for "*"..function tmux_focus_events#focus_gained:
line    8:

It is caused by matching all events with au * * ... Fixed by limiting: au CmdLineLeave * ...