Add spec comments to Header class

[Ethan-Arrowood]

It is useful to add comment references to the relevant parts of the whatwg fetch spec for the Headers class.

[ronag]

Also improve spec conformance, e.g. the errors we throw do not follow spec.

[ronag]

Is fill to loose in relation to the spec? i.e. does it allow "init" to be things that browser fetch wouldn't accept?

[Ethan-Arrowood]

Yes, flat array is not spec compliant; this was a point of discussion in the Node core PR.

It helps us support the raw header list we get from other Node http things as well as being faster than other header implementations

[ronag]

Can we explicitly document the differences?

[ronag]

I’m thinking if we could expose the non spec compliant things as exported helper functions. That way we don’t violate the spec.

[Ethan-Arrowood]

Now that we are going complete spec compliant, I agree.

[ronag]

I did some of this but there is still work to be done.

[ronag]

Also should probably remove the undici errors with e.g. TypeErrors.