Moved plugins into individual component module files which are
automatically required by lazy.nvim. Should make everything a tiny bit
more modular, or at least prepare the way for true modularity if I ever
have the time on my hands to ensure everything works with missing
modules.
Moved core settings into their own directory (`core`), and created a
`personal` folder which contains functions/plugins I wrote that do not
necessarily have to be their own imported plugin yet.
Finally, extended the utility functions a little, so we can detect if a
plugin exists and change e.g. key maps based on that (once again,
extending modularity a little more). Some simple attempts have been made
at that in the `mappings.lua` file, though it is nowhere near extensive
yet - most keymaps are still set regardless of plugin availability.
However, with this slimmer base to work off of, I feel more confident in
changing future things about this setup a little more ad-hoc without
having as many ripple repercussions as before.
