feat: api v9 and threads (#5570)

Co-authored-by: Noel <icrawltogo@gmail.com>
Co-authored-by: Amish Shah <dev@shah.gg>
Co-authored-by: Vlad Frangu <kingdgrizzle@gmail.com>
Co-authored-by: SynthGhost <60333233+synthghost@users.noreply.github.com>
Co-authored-by: SpaceEEC <24881032+SpaceEEC@users.noreply.github.com>
Co-authored-by: Elliot <elliot@maisl.fr>
Co-authored-by: Antonio Román <kyradiscord@gmail.com>
Co-authored-by: Sugden <28943913+NotSugden@users.noreply.github.com>

src/client/actions/ThreadMembersUpdate.js

@@ -0,0 +1,34 @@
+'use strict';
+
+const Action = require('./Action');
+const { Events } = require('../../util/Constants');
+
+class ThreadMembersUpdateAction extends Action {
+  handle(data) {
+    const client = this.client;
+    const thread = client.channels.cache.get(data.id);
+    if (thread) {
+      const old = thread.members.cache.clone();
+      thread.memberCount = data.member_count;
+
+      data.added_members?.forEach(rawMember => {
+        thread.members._add(rawMember);
+      });
+
+      data.removed_member_ids?.forEach(memberId => {
+        thread.members.cache.delete(memberId);
+      });
+
+      /**
+       * Emitted whenever members are added or removed from a thread. Requires `GUILD_MEMBERS` privileged intent
+       * @event Client#threadMembersUpdate
+       * @param {Collection<Snowflake, ThreadMember>} oldMembers The members before the update
+       * @param {Collection<Snowflake, ThreadMember>} newMembers The members after the update
+       */
+      client.emit(Events.THREAD_MEMBERS_UPDATE, old, thread.members.cache);
+    }
+    return {};
+  }
+}
+
+module.exports = ThreadMembersUpdateAction;