types(Client): fix isReady narrowing (#9828)

* types(Client): fix isReady narrowing

* chore: add requested changes

* chore: test both ready cases

---------

Co-authored-by: kodiakhq[bot] <49736102+kodiakhq[bot]@users.noreply.github.com>

Author: N1ckPro

packages/discord.js/typings/index.d.ts

@@ -955,6 +955,8 @@ export class Client<Ready extends boolean = boolean> extends BaseClient {
   private _eval(script: string): unknown;
   private _validateOptions(options: ClientOptions): void;
   private get _censoredToken(): string | null;
+  // This a technique used to brand the ready state. Or else we'll get `never` errors on typeguard checks.
+  private readonly _ready: Ready;
 
   public application: If<Ready, ClientApplication>;
   public channels: ChannelManager;

packages/discord.js/typings/index.test-d.ts

@@ -197,6 +197,12 @@ const client: Client = new Client({
   }),
 });
 
+if (client.isReady()) {
+  expectType<Client<true>>(client);
+} else {
+  expectType<Client>(client);
+}
+
 const testGuildId = '222078108977594368'; // DJS
 const testUserId = '987654321098765432'; // example id
 const globalCommandId = '123456789012345678'; // example id