Improve headers types

[StarpTech]

Headers supports an array of values new Headers({ "foo": ["a", "b"] }) but this isn't reflected in the types.
It results in { "foo": "a,b" }.

Would you accept a PR?

[Gozala]

Hey @StarpTech having more accurate types would be awesome, that said I'm not sure if diverging from the stock TS types would be a good idea and currently unfortunately this is what I see in TS playground

I think this should be fixed on TS side, this library simply tells TS that Headers come from globals (even though node mapped version does something else)

io/packages/fetch/src/lib.js

Line 3 in 3c20536

export const { Headers, Request, Response } = globalThis;