TODO: Properly support HTTP/3 Trailer

[jasnell]

HTTP/3 Trailing headers are not yet fully supported

[jasnell]

Starting to work through the details of how trailing headers should work on the API level, which will drive how they function on the lower bits...

Two specific bits:

• Ending a stream without sending trailing headers will close the writable side without sending any trailing headers.
• Calling submitTrailers() on an open stream will have the side effect of closing the writable side of the stream, so it should only be called when all data has been sent.

Therefore, there are two basic scenarios:

No trailers:

stream.write('hello ')
stream.end('world')

// Calling stream.submitTrailers() at this point would throw

With trailers:

stream.write('hello ')
stream.write('world')
stream.submitTrailers({ foo: 'bar' })  // Closes the stream! like calling end()

// Calling stream.write() at this point would throw

[jasnell]

Trailers are working as of #288 but there are still a number of API details to work through.