chore: add i18n_extract to package.json for openedx-translations (#1013)

This step is needed for openedx-translations repo to extract source files.

This contribution 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).

package.json

@@ -8,6 +8,7 @@
   ],
   "scripts": {
     "build": "fedx-scripts webpack",
+    "i18n_extract": "BABEL_ENV=i18n fedx-scripts babel src --quiet > /dev/null",
     "build:with-theme": "THEME=npm:@edx/brand-edx.org@latest npm run install-theme && fedx-scripts webpack",
     "check-types": "tsc --noemit",
     "lint": "fedx-scripts eslint --ext .js --ext .jsx .; npm run check-types",