Split up the URLPattern implementation (servo#36391)

The current implementation is already rather large at ~2.5k lines (~2k
LoC). There is still quite a lot of functionality left to implement, so
let's split it up while it's still manageable.

Testing: Covered by existing web platform tests

Signed-off-by: Simon Wülker <simon.wuelker@arcor.de>

Author: simonwuelker