* fix(app-router): router.prefetch throws on invalid URL
Match Next.js's createPrefetchURL behaviour: when the href cannot be
parsed via `new URL(href, window.location.href)`, throw the canonical
message `Cannot prefetch '<href>' because it cannot be converted to a
URL.` so the surrounding error boundary in the App Router can surface
the failure instead of silently swallowing it inside the async prefetch
IIFE.
Closes#1553
* fix(app-router): apply basePath before prefetch URL validation
Mirror Next.js's createPrefetchURL exactly by passing the href through
withBasePath before parsing. With a non-empty basePath, an otherwise
broken-looking href can be parseable (e.g. `new URL("/app///", origin)`
succeeds while `new URL("///", origin)` throws), so applying basePath
first matches upstream behaviour for basePath-configured apps.
James Anderson