This also required converting URLSearchParams::for_each and the callback
function it invokes to ThrowCompletionOr. With this, the ReturnType enum
used by WrapperGenerator is removed as all callers would be using
ReturnType::Completion.
The new URL built-in that will reside in URL.{h, cpp} will have an
URLSearchParams member, which means it has to include its header, and
as such URLSearchParams.h can't include URL's header, which it needs as
it uses the url_{encode, decode} functions.
This is a very partial implementation, as some features (like 2 of the
possible constructor types, iteration and the getAll method) are
missing, and other's are not implemented due to the currently missing
URL built-in.
