Refactor ReadableStream like done for WritableStream
<rdar://problem/102648417>
Pull request: https://github.com/WebKit/WebKit/pull/6786
Committed 257174@main (787ad7f7407a): <https://commits.webkit.org/257174@main> Reviewed commits have been landed. Closing PR #6786 and removing active labels.
Re-opened since this is blocked by bug 248636
Pull request: https://github.com/WebKit/WebKit/pull/12163
Committed 262590@main (647a58855607): <https://commits.webkit.org/262590@main> Reviewed commits have been landed. Closing PR #12163 and removing active labels.