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/by-name/treesitter/default.nix

@@ -431,7 +431,7 @@ helpers.neovim-plugin.mkNeovimPlugin {
   installPackage = false;
 
   extraConfig = cfg: {
-    extraConfigLua =
+    plugins.treesitter.luaConfig.content =
       # NOTE: Upstream state that the parser MUST be at the beginning of runtimepath.
       # Otherwise the parsers from Neovim takes precedent, which may be incompatible with some queries.
       (optionalString (cfg.settings.parser_install_dir != null) ''