nvim: Move util to core.util module

Moved all utility functions from their own directory into
the core functionality direcotyr as a single file.

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

@@ -60,7 +60,7 @@ local prose_plugs = {
 	{
 		"mickael-menu/zk-nvim",
 		config = function()
-			if require("util").is_available("which-key") then
+			if require("core.util").is_available("which-key") then
 				local prefix = require("which-key").register
 				prefix({ ["<leader>n"] = { name = "+notes" } })
 				prefix({ ["<localleader>n"] = { name = "+note" } })