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/pluginmanagers/lz-n.nix

@@ -190,7 +190,7 @@ nixvim.neovim-plugin.mkNeovimPlugin {
 
   extraConfig = cfg: {
     globals.lz_n = modules.mkAliasAndWrapDefsWithPriority id options.plugins.lz-n.settings;
-    extraConfigLua = mkIf (cfg.plugins != [ ]) ''
+    plugins.lz-n.luaConfig.content = mkIf (cfg.plugins != [ ]) ''
       require('lz.n').load( ${nixvim.toLuaObject cfg.plugins})
     '';
   };