tweak: jest loads .mjs last

This resolves an issue that caused jest tests to fail when depending on
a module that ships .mjs files (encountered via a transitive dep of
react-markdown).

See https://github.com/facebook/create-react-app/pull/4085 for context.

Test plan: I have a future commit which tests a file that depends on
react-markdown. The tests fail to run before this commit, and they run
without issue afterwards.

package.json

@@ -124,12 +124,12 @@
     },
     "moduleFileExtensions": [
       "web.js",
-      "mjs",
       "js",
       "json",
       "web.jsx",
       "jsx",
-      "node"
+      "node",
+      "mjs"
     ]
   },
   "files": [