Since this plugin is written in Vim9script, it won't work in any earlier
version. Add a guard to prevent the plugin from loading in earlier Vim
versions.

Clean up the plugin file and move the bulk of the work to autoload.

If the 'xkbswitch' command isn't available, we shouldn't be adding any
autocmds.

I assumed that I should be removing existing autocmds before adding new
ones. If you ran the `:InsertLayout` command more than once, I didn't
want old autocmds to stick around.

The line was commented out because it fails on the first call to
`:InsertLayout`, since at that point the `insert_layout` augroup doesn't
exist.

On closer inspection, however, it turns out that the old autocmds are
not retained when running `:InsertLayout` subsequent times. Rather, the
newly-defined ones are overwriting the old ones. So we don't need to
worry about removing the old autocmds before adding new ones.

This allows us to complete the command's keyboard layout identifier
making it easier to select one.

Add two new commands, `InsertLayout` and `InsertLayoutOff`, which allow
us to activate a different keyboard layout for insert mode and restore
the previous layout when leaving insert wode.

Works reasonably well, but needs a bit of cleaning up now.

This uses `xkbswitch` to change keyboard layouts on insert enter and
leave. For now it's only a proof of concept.

The plan is to make an `:InsertLayout` command that takes an argument
defining the keyboard layout to use in insert mode. When returning to
normal mode, it should switch back to the previous layout.

This uses https://github.com/myshov/xkbswitch-macosx/.