Parallelize initial page cache population (#134)

* Add p-limit package

* Populate page-cache in parallel instead of sequentially

Previously, all routes were set one-by-one. Instead, up to `availableParallelism` pages are now concurrently read from the filesystem and populated into the cache handler.

* Introduced a promise-backed latch so cache setup runs once before potential parallel work

Added `#configureTask` alongside the existing merged handler state and wired a new `#ensureConfigured()` helper that guards the original configuration routine. The `get`, `set`, and `revalidateTag` entry points now await `#ensureConfigured()`, so they can be executed in parallel after the one-time initialization completes without racing the user-provided `onCreation` hook

* Allow customizing the parallelism factor

Author: FabianKoder