Add Get Current User Guild Member Endpoint (#7)

for OAuth2 with guild.members.read scope

discord/discord-api-docs#4204

Not appear in the current documentation yet

Author: SQKo

src/Discord/Endpoint.php

@@ -197,6 +197,8 @@ class Endpoint
     public const USER_CURRENT_GUILDS = self::USER_CURRENT.'/guilds';
     // DELETE
     public const USER_CURRENT_GUILD = self::USER_CURRENT.'/guilds/:guild_id';
+    // GET
+    public const USER_CURRENT_MEMBER = self::USER_CURRENT.'/guilds/:guild_id/member';
     // GET, POST
     public const USER_CURRENT_CHANNELS = self::USER_CURRENT.'/channels';
     // GET