Add url tagged data type

[jclark]

This would be a tagged data type (like regexp), so that it can be serialized as a string in JSON. Relates to #1098.

Probably url is the right tag. From https://url.spec.whatwg.org/#goals:

Standardize on the term URL. URI and IRI are just confusing. In practice a single algorithm is used for both so keeping them distinct is not helping anyone. URL also easily wins the search result popularity contest.

[jclark]

Need to think about how interpolation (${...}) works. Currently it is defined to take values of the interpolated data type, but I think we also need it to takes strings. Interpolating a string would effectively quote it using the URL syntax (so it would works safely for query parameters).

[jclark]

A bigger issue would be to tie this into resource functions somehow (but resource paths aren't URLs).