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/none-ls/default.nix

@@ -147,7 +147,7 @@ helpers.neovim-plugin.mkNeovimPlugin {
       ];
 
       # We only do this here because of enableLspFormat
-      extraConfigLua = ''
+      plugins.none-ls.luaConfig.content = ''
         require("null-ls").setup(${helpers.toLuaObject setupOptions})
       '';
     };