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/ccc/default.nix

@@ -246,7 +246,7 @@ helpers.neovim-plugin.mkNeovimPlugin {
     # ccc requires `termguicolors` to be enabled.
     opts.termguicolors = lib.mkDefault true;
 
-    extraConfigLua = ''
+    plugins.ccc.luaConfig.content = ''
       ccc = require('ccc')
       ccc.setup(${helpers.toLuaObject cfg.settings})
     '';