Using '#Name' with exists() returns 1 if an autocmd has been defined
already for that event somewhere. What we want to do instead is check
that the event itself exists, which is done using '##Name'.
Avoids what seems to be an issue with nvim_win_set_config() and
WinEnter/WinLeave, which causes the context window to open at the
top-left corner of the screen when creating splits, rather than relative
to the current window with "relative = 'win'".
Opening the command-line window while the context window is open causes
errors.
You can only close the command-line window when it is open; attempting
to close other windows will throw E11.
See https://github.com/neovim/neovim/pull/10738
