Factor out basic email check (#10244)

src/email.ts

@@ -1,5 +1,6 @@
 /*
 Copyright 2016 OpenMarket Ltd
+Copyright 2023 The Matrix.org Foundation C.I.C.
 
 Licensed under the Apache License, Version 2.0 (the "License");
 you may not use this file except in compliance with the License.
@@ -22,5 +23,8 @@ const EMAIL_ADDRESS_REGEX = new RegExp(
 );
 
 export function looksValid(email: string): boolean {
+    // short circuit regex with this basic check
+    if (email.indexOf("@") < 1) return false;
+
     return EMAIL_ADDRESS_REGEX.test(email);
 }