[js][auth] add isSignInWithEmailLink method

lib/modules/auth/index.js

@@ -406,6 +406,18 @@ export default class Auth extends ModuleBase {
     );
   }
 
+  /**
+   * Checks if an incoming link is a sign-in with email link.
+   * @param {string} emailLink Sign-in email link.
+   */
+  isSignInWithEmailLink(emailLink: string): boolean {
+    return (
+      typeof emailLink === 'string' &&
+      emailLink.includes('mode=signIn') &&
+      emailLink.includes('oobCode=')
+    );
+  }
+
   /**
    * Completes the password reset process, given a confirmation code and new password.
    *