Add a new ExtraData enum. We carry it along with a custom property's
specified value in order to compute it, if it ends up being registered
as being able to contain URLs through Properties & Values. When the
specified value comes from a declaration, we keep track of the
associated UrlExtraData in the Specified variant. However, specified
values will also (implemented by a later patch in this series) be able
to come from animations: in that case we are able to carry along a copy
of the computed value in the Precomputed variant, to save us from
recomputation and also to save us from having to re-resolve the URL.

Add a new BorrowedExtraData enum, which is used for
BorrowedSpecifiedValues. We use BorrowedSpecifiedValues when resolving
custom properties; in the case that we inherit an untyped custom
property, we create one from its computed value, which is a token
stream. In this case we don't have a UrlExtraData or (Properties &
Values) ComputedValue to compute with, and in fact we don't want to
perform any computation. The BorrowedExtraData enum thus has an
additional InheritedUntyped variant for that case.

Pull out the css, first_token_type, and last_token_type fields of
SpecifiedValue, BorrowedSpecifiedValue, and ComputedValue into a
TokenStream struct (ComptuedValue then becomes a type alias of
TokenStream). We'll use the TokenStream to store computed token stream
values for typed custom properties: it wouldn't make sense to use
SpecifiedValue because we have no need of the additional information
(and that would also lead to properties_and_values::ComputedValue
containing a SpecifiedValue which contains an ExtraData which may
contain a properties_and_values::ComputedValue, which the compiler does
not like).

The properties_and_values module is addded by a later patch in this
series, so we omit the Precomputed variant for now.

Part 5 of a series of patches to implement the CSS Properties & Values API.