Add initial support for wasm-pack

[aDogCalledSpot]

Fixes https://github.com/taiki-e/cargo-llvm-cov/issues/337. Fixes https://github.com/taiki-e/cargo-llvm-cov/issues/221.

Requires https://github.com/rustwasm/wasm-bindgen/pull/3782.

Will also require a follow-up PR in wasm-pack but that will have trivial changes.

---

The changes made in this PR are still in a very rough state.

Before I turn this into a non-draft I'm mainly looking to get feedback about whether this is something you would be willing to include here.

Usage

Enable the unstable-coverage feature in the wasm-bindgen-test dependency.

cargo llvm-cov test --wasm --chrome --headless

Mixing WASM and non-WASM

Doing the following works:

cargo llvm-cov --no-report
cargo llvm-cov --no-report test --wasm --chrome --headless
cargo llvm-cov report

However, you need to do the following:

pub fn bool_to_str(b: bool) -> &'static str {
    if b {
        "true"
    } else {
        "false"
    }
}

pub fn bar(x: u32) -> u32 {
    if x == 0 {
        1
    } else {
        42
    }
}


fn main() {}

#[cfg(all(test, target_arch = "wasm32"))]
mod test {
    wasm_bindgen_test::wasm_bindgen_test_configure!(run_in_browser);
    use super::*;

    #[wasm_bindgen_test::wasm_bindgen_test]
    async fn test_foo() {
        assert_eq!(bool_to_str(true), "true");
    }
}

#[cfg(all(test, not(target_arch = "wasm32")))]
mod test {
    use super::*;

    #[test]
    fn test_bar() {
        assert_eq!(bar(0), 1);
    }
}

which is unfortunate.

I'll look into a bit and see what I find out.

@njelich do you maybe have any comments on this?