This patch implements the File API spec's supplemental steps for
document's "unloading document cleanup steps" so that we now remove blob
URLs associated with the document's relevant settings object when the
document is being unloaded.
Fixes two realm leaks when running our test suite.
This URL library ends up being a relatively fundamental base library of
the system, as LibCore depends on LibURL.
This change has two main benefits:
* Moving AK back more towards being an agnostic library that can
be used between the kernel and userspace. URL has never really fit
that description - and is not used in the kernel.
* URL _should_ depend on LibUnicode, as it needs punnycode support.
However, it's not really possible to do this inside of AK as it can't
depend on any external library. This change brings us a little closer
to being able to do that, but unfortunately we aren't there quite
yet, as the code generators depend on LibCore.
The Blob URL store is intended to be a singleton across all WebContent
instances. But for now, this implements a per-WebContent store, which
only lives as long as the WebContent process itself.
