dillon1000/react

[ReactFlightWebpackPlugin] Add support for .mjs file extension (#33028)

## Summary
Our builds generate files with a `.mjs` file extension. These are
currently filtered out by `ReactFlightWebpackPlugin` so I am updating it
to support this file extension.

This fixes https://github.com/facebook/react/issues/33155

## How did you test this change?
I built the plugin with this change and used `yalc` to test it in my
project. I confirmed the expected files now show up in
`react-client-manifest.json`