This is the core object behind a URL pattern which when constructed
can be used for matching the pattern against URLs.
However, the implementation here is missing key functions such as
the constructor and the 'test'/'exec' functions as that relies on
a significant amount of supporting URLPattern infrastructure such
as two different parsers and a tokenizer.
However, this is enough for us to implement some more of the IDL
wrapper layer of this specification.
A URL pattern consists of components such as the 'port', 'password'
'hostname', etc. A component is compiled from the input to the
URLPattern constructor and is what is used for matching against
URLs to produce a match result.
This is also where the regex dependency is introduced into LibURL
to support the URLPattern implementation.
This lets us move a few Host-related functions (like serialization and
checks for what the Host is) into Host instead of having them dotted
around the codebase.
For now, the interface is still very Variant-like, to avoid having to
change quite so much in one go.
