We've had many issues with WebM support and specifically the libvpx library
over the years, mostly due to its poor integration in Godot's buildsystem,
but without anyone really interested in improving this state.
With the new GDExtensions in Godot 4.0, we intend to move video decoding to
first-party extensions, and this would likely be done using something like
libvlc to expose more codecs.
Removing the `webm` module means we can remove libsimplewebm, libvpx and
opus, which we were only used for that purpose. Both libvpx and opus were
fairly complex pieces of the buildsystem, so this is a nice cleanup.
This also removes the compile-time dependency on `yasm`.
Fixes lots of compilation or non-working WebM issues which will be linked
in the PR.
For the time being we don't support writing a description for those, preferring
having all details in the method's description.
Using self-closing tags saves half the lines, and prevents contributors from
thinking that they should write the argument or return documentation there.
We already removed it from the online docs with #35132.
Currently it can only be "Built-In Types" (Variant types) or "Core"
(everything else), which is of limited use.
We might also want to consider dropping it from `ClassDB` altogether
in Godot 4.0.
