This reverts commit 92b905580bffb55d884ff07bcec660631304332b.

The file is generated at build time. It was kept around for
Rebar compatibility. Since then we have been publishing
releases to Hex so Rebar arguably doesn't need to fetch
the project from Git.

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.