fix: typings for 'Message' (#5518)

typings/index.d.ts

@@ -996,6 +996,7 @@ declare module 'discord.js' {
     public content: string;
     public readonly createdAt: Date;
     public createdTimestamp: number;
+    public readonly crosspostable: boolean;
     public readonly deletable: boolean;
     public deleted: boolean;
     public readonly editable: boolean;