fix!: immutable headers guards for incoming response and downstream request

[guybedford]

This implements immutable headers guards for the fetch event request, and any fetch() responses.

With this change, trying to set a header on one of these will throw a TypeError as is correct per the fetch and service worker specs.

Resolves #992.

[guybedford]

This should also reintroduce the "fetch/api/response/response-headers-guard.any.js" WPT test.

[guybedford]

This can be done as a minor since there is no functional change, so long as it rebased in a way that respects the Candidate Response phase of the Response permitting mutations (i.e. the definition of is_upstream needs to be tweaked, and the guard needs to change during the lifecycle).