Buffer streamed HTML responses in the service worker

[adamziel]

What it does

Buffers text/html response bodies in the service worker before returning them to the browser. Non-HTML responses (JSON, images, file downloads) continue streaming normally.

Rationale

When streaming HTML, Chrome view transitions are animating to the partially streamed state. This makes the UI elements appear one by one in a fast progression and feels weird. Most servers buffer the HTML responses, let's just do the same in Playground. Perhaps, in a follow-up, we could figure how to buffer every PHP response by defaul unless flush() is called.

Before merging, let's confirm this solves the problem.