fix(types): Change type to allow autocompletion and existence checking (#1368)

Author: robwalkerco

src/define-messages.ts

@@ -6,8 +6,12 @@ import {MessageDescriptor} from './types';
  * See the accompanying LICENSE file for terms.
  */
 
-export default function defineMessages(
-  messageDescriptors: Record<string, MessageDescriptor>
+type Messages<Names extends keyof any = string> = {
+  [key in Names]: MessageDescriptor;
+};
+
+export default function defineMessages<Names extends keyof any>(
+  messageDescriptors: Messages<Names>
 ) {
   // This simply returns what's passed-in because it's meant to be a hook for
   // babel-plugin-react-intl.