Adds a new `onEditorMount` callback to the store, allowing store
creators to do things like registering bookmark handlers. We use this in
the new demo hook.
This also renames `useRemoteSyncClient` to `useMultiplayerSync`, and
`useRemoteSyncDemo` to `useMultiplayerDemo`.
Closes TLD-2601
### Change type
- [x] `api`
Adds an assets server to the demo worker, and reworks the existing asset
server to use the same code. There are a few simplifications to the code
due to some DX improvements working with R2 and caches. I also removed
the `HEAD` request from the assets server: i took a look at our logs and
it's not actually used at all.
This also fixes an issue where users could overwrite the contents of the
asset uploads bucket.
### Change type
- [x] `other`
Sets up preview deploys etc. for bemo worker.
There's enough going on here that I wanted to make it its own PR. I'll
rework david's spike on top of it once it's landed.
### Change Type
- [x] `internal` — Does not affect user-facing stuff
- [x] `chore` — Updating dependencies, other boring stuff
---------
Co-authored-by: David Sheldrick <d.j.sheldrick@gmail.com>
