chore(client-entry): Slight tweaks around entry.client handling

[Tobbe]

I was trying to better understand how entry.client.tsx was used during SSR, and when following the code I found a few things I wanted to change to, in my opinion, make the code easier to read, understand and follow.

Even though entry.server.tsx is the main entry point for SSR, we still need to read the client build manifest and find entry.client.tsx to get the correct links to insert for the initial CSS files the browser will eventually render when the finalized html output is being streamed to the browser. We also need it to tell React what JS bundle contains hydrateRoot when it'll eventually get to hydrating things in the browser