WEBTRANSPORT -> WT in most places. Only ENABLE_WEBTRANSPORT
remains (since it is a draft-02 setting that was later removed)
as well as internal functions and atoms about stream headers,
for greater clarity.

This adds support for WebTransport HTTP/3 settings, error codes,
headers, stream types as well as capsules, with an initial
implementation of the Capsule protocol.

This allows cleaning up timers and avoids receiving
stray timeout messages when reusing a process. This
is otherwise optional since the termination of a
process cleans up the timers.

This is to be used primarily by a client before initiating a new stream
so that it can respect the setting from the server.