feat(gateway): add stopGateway function (#2078)

* Move the shard stopping to gatewayManager in order to stop the gateway standalone.

* Update gateway/gateway_manager.ts

Co-authored-by: ITOH <to@itoh.at>

* Update gateway/stopGateway.ts

Co-authored-by: ITOH <to@itoh.at>

* Update gateway/stopGateway.ts

* Update gateway/stopGateway.ts

* Update gateway/stopGateway.ts

Co-authored-by: ITOH <to@itoh.at>
Co-authored-by: Skillz4Killz <23035000+Skillz4Killz@users.noreply.github.com>

gateway/stopGateway.ts

@@ -0,0 +1,16 @@
+/** Use this function to stop the gateway properly */
+export async function stopGateway(gateway: GatewayManager, code = 3061, reason = "Discordeno Testing Finished! Do Not RESUME!") {
+
+    // STOP WS
+    gateway.shards.forEach((shard) => {
+        clearInterval(shard.heartbeat.intervalId);
+        gateway.closeWS(
+          shard.ws,
+          code,
+
+          reason,
+        );
+      });
+
+    await delay(5000);
+}