[Flight] Add webpack plugin build (#18485)

* Eject CRA from Flight

We need to eject because we're going to add a custom Webpack Plugin.

We can undo this once the plugin has upstreamed into CRA.

* Add Webpack plugin build

I call this entry point "webpack-plugin" instead of "plugin" even though
this is a webpack specific package. That's because there will also be a
Node.js plugin to do the server transform.

* Add Flight Webpack plugin to fixture

* Rm UMD builds

* Transform classes

* Rename webpack-plugin to plugin

This avoids the double webpack name. We're going to reuse this for both
server and client.