Implement the getSetCookie method of Headers

[ushiboy]

I've implemented the getSetCookie method of Headers. The HeaderList data structure has been changed accordingly.

See here for the specification of the getSetCookie method.
https://developer.mozilla.org/en-US/docs/Web/API/Headers/getSetCookie

With this fix, the header-setcookie.any.html test is resolved except for a failure due to Response not being implemented.

before

  1) web-platform-tests
       fetch/api/headers
         header-setcookie.any.html:
     18/24 errors in test:

Failed in "Headers iterator does not combine set-cookie headers":
assert_equals: Array length is not equal expected 2 but got 1
...

after

  1) web-platform-tests
       fetch/api/headers
         header-setcookie.any.html:
     1/24 errors in test:

Failed in "Set-Cookie is a forbidden response header"
Response is not defined

[ushiboy]

Why are CI tests for latest failing?

The overwritten-customElements-global.html is set up to expect the test to fail, but since node.js v21.7, the test seems to succeed.

This can be resolved by setting to-run.yaml to overwritten-customElements-global.html: [flaky, Unknown], but I cannot determine if that should be included in this PR.