Add proper support for "Parsing Structured Fields" from RFC8941 for both "item" and "dictionary" types. The spec also supports the "list" type, but I didn't add it yet since we don't have any users yet.

The "item" type is used to parse the Cross-Origin-Opener-Policy & Cross-Origin-Embedder-Policy headers:
- https://html.spec.whatwg.org/multipage/origin.html#obtain-coop (step 3)
- https://html.spec.whatwg.org/multipage/origin.html#obtain-an-embedder-policy (step 3)

The "dictionary" type is used to parse the Reporting-Endpoints header:
- https://w3c.github.io/reporting/#process-header (step 2)

We had partial support in 2 different places with no code sharing. We know have near complete support with code sharing.

This will allow us to implement support for the Permission-Policy header more easily in the near future:
- https://www.w3.org/TR/permissions-policy-1/#algo-process-response-policy (step 1)