CMakeLists: Add WASM_ASSERTIONS option

[fwcd]

This option will add additional checks, including stack overflow protection, which are disabled by default when building with optimizations (which we almost always do).

For more details, see https://emscripten.org/docs/porting/Debugging.html#compiler-settings

[fwcd]

Hm, with assertions enabled we unfortunately get a lot of errors of the form

Cannot have multiple async operations in flight at once

Sadly, it is pretty hard to debug where these calls are coming from, i.e. whether it's something in Qt or in Mixxx, since we do not get a stack trace... This results in Mixxx not being able to launch at all.

Marking this as a draft for now.

[fwcd]

We'll disable this option for now since it prevents Mixxx from launching currently (see this post for details, which is likely related to emscripten-core/emscripten#18412 and QTBUG-102827), but may be valuable for debugging in the future.

Marking as ready-for-review since it should be safe to merge (it doesn't change anything about the default config).

[daschuer]

LGTM, thank you.