dillon1000/react

Update Flight Fixture to "use client" instead of .client.js (#26118)

This updates the Flight fixture to support the new ESM loaders in newer
versions of Node.js.

It also uses native fetch since react-fetch is gone now. (This part
requires Node 18 to run the fixture.)

I also updated everything to use the `"use client"` convention instead
of file name based convention.

The biggest hack here is that the Webpack plugin now just writes every
`.js` file in the manifest. This needs to be more scoped. In practice,
this new convention effectively requires you to traverse the server
graph first to find the actual used files. This is enough to at least
run our own fixture though.

I didn't update the "blocks" fixture.

More details in each commit message.