Switch to putting config in a directory

[purefn]

Instead of using wrapRc to reference to add the -u argument to neovim, we add the directory to the XDG_CONFIG_DIRS, which neovim will read on startup.

This allows us to do a few nice things:

• Write commonly used code in modules. This is used to to create an internal module, flake/lsp, which contains the functions used for on_attach and the capabilities. Users can also add their own modules via vim.lua.modules.mymodule = "<code>";.
• Put language specific code in ftplugin/<lang>.lua. The nix config was migrated to use this approach as an example.

[jordanisaacs]

Sorry for the delayed response. This is an interesting feature which I am open to merging as an option that is by default disabled. This is because I see the primary use case of this flake being completely reproducible configurations that do not leak into your environment, hence the use of wrapRc.

But I see why someone may want to use XDG_CONFIG_DIRS, which is why I would be open to merging it as a configurable option.