lib/neovim-plugin: Add lua configuration scoped to the plugin

This commit adds a `plugins.<name>.luaConfig` section controlling the
plugin specific configuration.

The section contains the internal `init` option, containing the plugin's
initialization code.

It also contains the public `pre` and `post` options, that allow to add
code before & after the `init` section

Finally, it contains the `final` option, being the concatenation of the
three previous options.

plugins/colorschemes/base16/default.nix

@@ -179,7 +179,7 @@ lib.nixvim.neovim-plugin.mkNeovimPlugin {
     # `settings` can either be passed to `with_config` before calling `setup`,
     # or it can be passed as `setup`'s 2nd argument.
     # See https://github.com/RRethy/base16-nvim/blob/6ac181b5733518040a33017dde654059cd771b7c/lua/base16-colorscheme.lua#L107-L125
-    extraConfigLuaPre = ''
+    colorschemes.base16.luaConfig.content = ''
       do
         local base16 = require('${luaName}')
         base16.with_config(${toLuaObject cfg.settings})