fix: let make extract_translations find messages (#290)

Otherwise it'll complain for not finding any message.

`defineMessage` ensures that React i18n static code collector is able to
find the messages.

References
----------

This pull request is part of the [FC-0012 project](https://openedx.atlassian.net/l/cp/XGS0iCcQ) which is sparked by the [Translation Infrastructure update OEP-58](https://open-edx-proposals.readthedocs.io/en/latest/architectural-decisions/oep-0058-arch-translations-management.html#specification).

Check the links above for full information about the overall project.

src/editors/sharedComponents/SourceCodeModal/messages.js

@@ -1,4 +1,7 @@
-export const messages = {
+import { defineMessages } from '@edx/frontend-platform/i18n';
+
+const messages = defineMessages({
+
   saveButtonLabel: {
     id: 'authoring.texteditor.sourcecodemodal.next.label',
     defaultMessage: 'Save',
@@ -9,5 +12,6 @@ export const messages = {
     defaultMessage: 'Edit Source Code',
     description: 'Title for the source code editor',
   },
-};
+});
+
 export default messages;