Make default policy available to tools

[mikesamuel]

Tools like a template language might want to apply a policy to string inputs, akin to applying the default policy, for example to vet strings that reach <a href={{x}}>. They might benefit from being able to default to the document's default policy.

Maybe expose an [Unforgeable] property TrustedTypes.defaultPolicy with type null | TrustedTypesPolicy.

[koto]

That sounds useful, can you add it to the spec?