[qtbase] Add asyncify feature

[fwcd]

This adds support for building Qt with Emscripten's asyncify when targeting WebAssembly as per

• https://doc.qt.io/qt-6/wasm.html#qt-configure-options-reference
• https://github.com/qt/qtbase/blob/210f2004df/cmake/configure-cmake-mapping.md

Marking this as a draft for now, since I haven't figured out whether this is actually needed (just linking the downstream app with -sASYNCIFY seems to work without this too), though it looks like it forces the downstream app to use asyncify.

Feedback is of course appreciated.

Checklist

• Changes comply with the maintainer guide.
• SHA512s are updated for each updated download.
• The "supports" clause reflects platforms that may be fixed by this new version.
• Any fixed CI baseline entries are removed from that file.
• Any patches that are no longer applied are deleted from the port's directory.
• The version database is fixed by rerunning ./vcpkg x-add-version --all and committing the result.
• Only one version is added to each modified port's versions file.

[FrankXie05]

@fwcd Are you still continuing this PR? :)