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.

nvim/.config/nvim/lua/plugins/prose.lua

@@ -0,0 +1,56 @@
+local writing_ft = { "quarto", "pandoc", "markdown", "text", "tex" }
+
+return {
+	-- UI improvements
+	-- provide distraction free writing
+	{ "folke/zen-mode.nvim", config = true, event = "VeryLazy" },
+	-- provide even distraction free-er writing (lowlight paragraphs)
+	{ "folke/twilight.nvim", event = "VeryLazy" },
+	-- enable 'speed-reading' mode (bionic reading)
+	{
+		"JellyApple102/easyread.nvim",
+		config = true,
+		ft = writing_ft,
+		cmd = "EasyreadToggle",
+	},
+	{
+		"andrewferrier/wrapping.nvim",
+		config = function()
+			require("wrapping").setup({
+				create_keymappings = false,
+				notify_on_switch = false,
+			})
+		end,
+	},
+	-- generate an auto-updating html preview for md files
+	{
+		"iamcco/markdown-preview.nvim",
+		build = function()
+			vim.fn["mkdp#util#install"]()
+		end,
+		ft = writing_ft,
+	},
+
+	-- bring zettelkasten commands
+	{
+		"mickael-menu/zk-nvim",
+		config = function()
+			require("zk").setup({ picker = "telescope" })
+		end,
+	},
+	-- simple static markdown linking and link following using zettel IDs
+	{ "marty-oehme/zettelkasten.nvim", ft = writing_ft, event = "VeryLazy" },
+
+	-- syntax highlighting for markdown criticmarkup (comments, additions, ...)
+	{ "vim-pandoc/vim-criticmarkup", ft = writing_ft },
+	-- inline display of latex formulas
+	-- TODO always demands latex treesitter to be installed even if it is
+	-- TODO always turns softwrapped lines off on exiting insert mode
+	--{
+	--"jbyuki/nabla.nvim",
+	--ft = writing_ft,
+	--config = function()
+	--require("nabla").enable_virt({ autogen = true, silent = true })
+	--end,
+	--},
+}