Add stage instance & guild template endpoint (#8)

I previously forgotten the STAGE_INSTANCES endpoint

Author: SQKo

src/Discord/Endpoint.php

@@ -186,9 +186,18 @@ class Endpoint
     // GET, DELETE
     public const INVITE = 'invites/:code';
 
+    // POST
+    public const STAGE_INSTANCES = 'stage-instances';
     // GET, PATCH, DELETE
     public const STAGE_INSTANCE = 'stage-instances/:channel_id';
 
+    // GET, POST
+    public const GUILDS_TEMPLATE = self::GUILDS.'/templates/:template_code';
+    // GET, POST
+    public const GUILD_TEMPLATES = self::GUILD.'/templates';
+    // PUT, PATCH, DELETE
+    public const GUILD_TEMPLATE = self::GUILD.'/templates/:template_code';
+
     // GET, PATCH
     public const USER_CURRENT = 'users/@me';
     // GET