Per-branch workers

[iansk]

Implement per-branch workers to enable trouble-free, it-just-works previews.

Currently, if the correct book.yml isn't in place in main in the repo, preview pages can fail to load. Although there's a well-known work-around, the users for previews are often not very technical and/or don't know about, or even care about how the backend works. In this case, it takes a number of non-obvious steps to get previews working.

We're open to a paid CloudFlare workers plan, if necessary.

[iansk]

Example of a use case where the single worker system gets tripped up:

• Renaming product version directory from 30-xx to 30 - With this rename, previews completely break. The book.yml needs to be checked into main for it to work. For a normal writer, this isn't a straight forward operation, and they'll easily get tripped up.