Provide Qt for WebAssembly binary package variants

Currently, the CI system build Qt for WebAssembly using a single configuration only. We'd like to expand on this and provide multiple build variants:

• wasm-st (default, current package)
• wasm-mt (multithreaded)
• wasm-asyncify
• wasm-simd

Maybe also (some of) the combinations:

• wasm-mt-asyncify

The build variants should be offered as install variants in the Qt installer.

wasm-asyncify could also be useful for running the Qt auto-tests, since many of the tests are synchronous (with waitForExposed() calls, etc).