Factor out basic email check (#10244)

test/email-test.ts

@@ -0,0 +1,32 @@
+/*
+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.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+import { looksValid } from "../src/email";
+
+describe("looksValid", () => {
+    it.each([
+        ["", false],
+        ["alice", false],
+        ["@", false],
+        ["@alice:example.com", false],
+        ["@b.org", false],
+        ["alice@example", false],
+        ["a@b.org", true],
+        ["alice@example.com", true],
+    ])("for »%s« should return %s", (value: string, expected: boolean) => {
+        expect(looksValid(value)).toBe(expected);
+    });
+});