Transition to wasm-bindgen-test/unstable-test-coverage

[daxpedda]

Now that the #[coverage] attribute is stabilized (rust-lang/rust#130766), we can remove the hacks required to make this work and transition to a proper crate feature.

While this should be stabilized in Rust v1.85, we should wait until the release until actually encoding this information into WBG. Who knows, it might be delayed for one reason or another.

With LLVM 20 this shouldn't require any further hacks to actually run test coverage and profiling should be supported as well. While it will still require nightly Rust because of -Zno-profiler-runtime, everything should be covered from wasm-bindgens side. Before stabilizing it we should also transition the wording from test coverage to profiling to be more accurate.

[daxpedda]

Setting to draft until Amanieu/minicov#28 is resolved. Right now enabling the crate feature will emit .profraw files even when not instrumenting the Wasm module with -Cinstrument-coverage.

This kinda defeats the purpose of this PR, where users could enable the crate feature in dev-dependencies and not requiring more flags to get test coverage going. But the current situation would require using --features wasm-bindgen-test/unstable-test-coverage.

So we are just trading a cfg with a crate feature, slightly better, but not by much.