Refactor the monolithic `lvim.lang` design into a more modular approach.
IMPORTANT: run `:LvimUpdate` in order to generate the new ftplugin template files.
* Define core/builtins, streamline status_color interface
* Encapsulate configuration in its own module
* Add fallback to lv-config.lua
* Rectify settings loading order to allow overriding vim options
* Move default-config into config/ module
* replace uv.fs_stat with utils.is_file