Serve public files given to the runtime from the Service Worker

Goal: Serve files under the public directory (/public/) at the preview root when they are passed to ready() or added with addFile(), as Vite does.
Current gap: The Service Worker builds its list of public files once, when its dev server starts. At that point its virtual filesystem holds only /public/.gitkeep. The list is never updated, so servePublicMiddleware skips every other public file.
Effect: A request for a public file gets the SPA fallback index.html with status 200 instead of the file. For example, /__preview__/logo.svg does not return /public/logo.svg.
Request: Keep the Service Worker's public file list in sync with its virtual filesystem, as the Web Worker already does in setupHMR().
Verification status: Reproduced in the browser on the integration playground (see Details).
Related: Found while implementing #42 (#61). The README and the JSDoc of addFile() describe this as a current limitation. That note should be removed with the fix.

Current implementation, verification, requested behavior, and acceptance criteria

Reviewed at ba60d7b.

When the Service Worker script is evaluated, its entry writes /public/.gitkeep and starts the dev server. The runtime sends the files given to ready() afterwards, with V_FS_INIT.
createServer() from @vrowzer/vite-dev-server/service-worker calls initPublicFiles() during startup and waits for it. initPublicFiles() reads the public directory once.
The server registers servePublicMiddleware(server, publicFiles), which skips any path that is not in the set.
The server receives the file sync subscriber's watcher through watcherFactory, but nothing listens to its events, so the set never changes.
The Web Worker updates its own set on add and unlink, as Vite does. The Web Worker does not serve public files, though.

I checked this on the integration playground (packages/vrowzer/integration/playground) with temporary changes, which were reverted afterwards.

After ready(), addFile('/public/file-sync-public.txt', 'public text'), then fetch('/__preview__/file-sync-public.txt') from the preview: status 200, and the body is the transformed index.html.
With '/public/initial-public.txt' added to the files given to ready(), fetching /__preview__/initial-public.txt gave the same result.

Public files given to ready(), and files added under /public/ later, are served at the preview root.
After a public file is deleted, it is no longer served.
When addFile() or deleteFile() resolves, the change is already visible to requests (Return promises when file changes are visible to preview reads #42). The Service Worker acknowledges a file change right after applying it, so the list should be updated synchronously when the watcher event is emitted.

A public file given to ready() is served with its content.
A public file added with addFile() is served once the promise resolves. After deleteFile() resolves, it is no longer served.
The limitation is removed from the README and the JSDoc of addFile().

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