fix(typings): Restore inadvertently-deleted typings/msg/msg.d.ts; add TS import test (#7955)

* test(typings): Add langfile (msg) import + typings test

* fix(typings): Restore inadvertently-deleted typings/msg/msg.d.ts

  Fixes #7952.

  The main typings file for langfiles, typings/msg/msg.d.ts, was
  inadvertently deleted in PR #7822.

  This was part of a well intentioned attempt to remove spurious
  files from typings/msg/ that do not correspond to published
  langfiles, but this file should have been retained because
  msg.d.ts is reexported by all the other *.d.ts files in this
  directory.

tests/typescript/src/msg.ts

@@ -0,0 +1,20 @@
+/**
+ * @license
+ * Copyright 2022 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Test: Should be able to import messages and verify their type.
+ * Test at least one language other than English!
+ */
+
+import * as en from 'blockly-test/msg/en';
+import * as fr from 'blockly-test/msg/fr';
+
+let msg: {[key: string]: string};
+msg = fr;
+msg = en;
+
+// Satisfy eslint that msg is used.
+console.log(msg['DIALOG_OK']);