The OptionSet autocommand does not fire until Vim has finished starting,
so setting 'background' before the VimEnter event would not fire the
OptionSet event. The prior implementation also waited until VimEnter to
set 'background', so this was a regression introduced when moving
background detection into Lua.

This fixes an error that can occur in certain pathological cases when
the autocommand fires at just the right time such that it attempts to
close the timer after the timer has already exited, but before the
scheduled callback has fired.

We now let the timer continue to run even when the autocommand deletes
itself to avoid having to repeat the cleanup code multiple times. There
is no harm in letting the timer execute if the autocommand does not
exist, as the pcall will catch the error.

Move default mappings and autocommands into a separate module and add
comments and docstrings to document each of the defaults.