nvim: Restructure lua dir

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.

desktop/.config/user-dirs.dirs

@@ -0,0 +1,15 @@
+# This file is written by xdg-user-dirs-update
+# If you want to change or add directories, just edit the line you're
+# interested in. All local changes will be retained on the next run.
+# Format is XDG_xxx_DIR="$HOME/yyy", where yyy is a shell-escaped
+# homedir-relative path, or XDG_xxx_DIR="/yyy", where /yyy is an
+# absolute path. No other format is supported.
+# 
+XDG_DESKTOP_DIR="$HOME/desktop"
+XDG_DOCUMENTS_DIR="$HOME/documents"
+XDG_DOWNLOAD_DIR="$HOME/downloads"
+XDG_MUSIC_DIR="$HOME/media/audio/music"
+XDG_PICTURES_DIR="$HOME/pictures"
+XDG_PUBLICSHARE_DIR="$HOME/"
+XDG_TEMPLATES_DIR="$HOME/"
+XDG_VIDEOS_DIR="$HOME/"