feat: add guild.fetchVanityCode() (#2732)

* Error for guild.fetchVanityURL feature not enabled

* added GET/guilds/{guild.id}/vanity-url endpoint

* fix: code conventions

* adopted suggestion

* Changed error message according to change request

* Renamed method

to indicate that only the code is fetched, not the entire url.

* Update Guild.js

src/errors/Messages.js

@@ -97,6 +97,8 @@ const Messages = {
   EMOJI_MANAGED: 'Emoji is managed and has no Author.',
 
   REACTION_RESOLVE_USER: 'Couldn\'t resolve the user ID to remove from the reaction.',
+
+  VANITY_URL: 'This guild does not have the VANITY_URL feature enabled.',
 };
 
 for (const [name, message] of Object.entries(Messages)) register(name, message);