Change formatCount impl to use Intl.NumberFormat (#11379)

* Change formatCount impl to use Intl.NumberFormat

* Update formatCount JSDoc description

test/utils/FormattingUtils-test.ts

@@ -0,0 +1,42 @@
+/*
+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 { formatCount, formatCountLong } from "../../src/utils/FormattingUtils";
+
+jest.mock("../../src/dispatcher/dispatcher");
+
+describe("FormattingUtils", () => {
+    describe("formatCount", () => {
+        it.each([
+            { count: 999, expectedCount: "999" },
+            { count: 9999, expectedCount: "10K" },
+            { count: 99999, expectedCount: "100K" },
+            { count: 999999, expectedCount: "1M" },
+            { count: 9999999, expectedCount: "10M" },
+            { count: 99999999, expectedCount: "100M" },
+            { count: 999999999, expectedCount: "1B" },
+            { count: 9999999999, expectedCount: "10B" },
+        ])("formats $count as $expectedCount", ({ count, expectedCount }) => {
+            expect(formatCount(count)).toBe(expectedCount);
+        });
+    });
+
+    describe("formatCountLong", () => {
+        it("formats numbers according to the locale", () => {
+            expect(formatCountLong(1000)).toBe("1,000");
+        });
+    });
+});