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/which-key/default.nix

@@ -596,7 +596,7 @@ lib.nixvim.neovim-plugin.mkNeovimPlugin {
         ''
       ];
 
-      extraConfigLua = lib.optionalString opt.registrations.isDefined ''
+      plugins.which-key.luaConfig.content = lib.optionalString opt.registrations.isDefined ''
         require("which-key").register(${toLuaObject cfg.registrations})
       '';
     };