Use `import type` to import the Message interface

Use TypeScript 3.8+ `import type` to import the Message interface. This
addresses issues with the type being removed by transpilation, which
thus prevents "export 'Message' was not found" warnings when running
`nx serve api`

apps/api/src/app/app.gateway.ts

@@ -1,5 +1,5 @@
+import type { Message } from '@chat-room/api-interfaces';
 import {
-  Message,
   MESSAGE_TO_CLIENT,
   MESSAGE_TO_SERVER,
 } from '@chat-room/api-interfaces';