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/onedark.nix

@@ -34,7 +34,7 @@ lib.nixvim.neovim-plugin.mkNeovimPlugin {
   callSetup = false;
   colorscheme = null;
   extraConfig = cfg: {
-    extraConfigLuaPre = ''
+    colorschemes.onedark.luaConfig.content = ''
       _onedark = require('onedark')
       _onedark.setup(${lib.nixvim.toLuaObject cfg.settings})
       _onedark.load()