Implement streaming using responseType=stream (Streams API)

[Rob--W]

We should use the Streams API instead of range requests when it becomes available. A Firefox-specific patch to support the Streams API was added here #5263.

When a concrete implementation of the Streams API appears in other browsers (e.g. Chrome), then we should support that too. Here are some relevant bugs on Chromium's issue tracker:

• https://crbug.com/401396 Add a response type for ReadableStream to XMLHttpRequest
• https://crbug.com/240603 Streams API

[timvandermeij]

Is this still relevant? I think we have streams API support now, at least through a polyfill if I'm not mistaken. Both upstream issues are fixed too.

[Rob--W]

Fixed by #8768.