Remove readonly from RequestInit typings

[NMinhNguyen]

Bug Description

It'd be good to remove readonly modifier from RequestInit typings to match https://github.com/microsoft/TypeScript/blob/95731f03979985609c9fd662f179be5b9cea3470/lib/lib.dom.d.ts#L1470. This would allow for RequestInit objects to be conditionally mutated like so https://github.com/ferdikoomen/openapi-typescript-codegen/blob/0f11ea84a9ca9867bb72700f468fa21d8085ce86/src/templates/core/fetch/sendRequest.hbs#L20

I'm happy to raise a PR for this if there are no objections.

Reproducible By

If you try to mutate a RequestInit object, TypeScript will say something like:

Cannot assign to 'credentials' because it is a read-only property. ts(2540)

Expected Behavior

No error.

Logs & Screenshots

Environment

Additional context

[KhafraDev]

PR welcome, make sure to include tests. We use tsd for testing types.

[NMinhNguyen]

Thanks @KhafraDev, let me know if the test in #1446 isn't enough and I'll see what I can do