build: Compile prql-js for wasm only

[max-sixty]

While attempting to look at #3098, I tried to look at what was causing the builds the second time around.

Here's a fish script (easy to adjust for bash):

begin;
    echo 'Running: cargo +nightly clean'; cargo +nightly clean ;
    echo 'Running first build'; cargo +nightly build --verbose --all-targets;
    echo 'Running second build'; cargo +nightly build --verbose --all-targets -p prqlc;
end &| tee build.log

Then there's two lines in build.log for the same dependency, one from the first; one from the second:

    Running `/Users/maximilian/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustc --crate-name serde /Users/maximilian/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.163/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --cfg 'feature="default"' --cfg 'feature="derive"' --cfg 'feature="serde_derive"' --cfg 'feature="std"' -C metadata=5e334edc106c7255 -C extra-filename=-5e334edc106c7255 --out-dir /Users/maximilian/workspace/prql/target/debug/deps -L dependency=/Users/maximilian/workspace/prql/target/debug/deps --extern serde_derive=/Users/maximilian/workspace/prql/target/debug/deps/libserde_derive-6e3b03dd545079c5.dylib --cap-lints allow`
    Running `/Users/maximilian/.rustup/toolchains/nightly-aarch64-apple-darwin/bin/rustc --crate-name serde /Users/maximilian/.cargo/registry/src/index.crates.io-6f17d22bba15001f/serde-1.0.163/src/lib.rs --error-format=json --json=diagnostic-rendered-ansi,artifacts,future-incompat --crate-type lib --emit=dep-info,metadata,link -C embed-bitcode=no -C debuginfo=2 -C split-debuginfo=unpacked --cfg 'feature="alloc"' --cfg 'feature="default"' --cfg 'feature="derive"' --cfg 'feature="serde_derive"' --cfg 'feature="std"' -C metadata=7fbd537ea76b6b2f -C extra-filename=-7fbd537ea76b6b2f --out-dir /Users/maximilian/workspace/prql/target/debug/deps -L dependency=/Users/maximilian/workspace/prql/target/debug/deps --extern serde_derive=/Users/maximilian/workspace/prql/target/debug/deps/libserde_derive-6e3b03dd545079c5.dylib --cap-lints allow`

These have different features — only one has alloc. And then running:

cargo tree -e features --target=(default-target) -i serde

...helps find the thing that's generating the request for the alloc feature — in this case wasm-bindgen.

This seems like a very labor-intensive way of trying to unify dependencies between each crate & the workspace. But I'm including the first fix, which is to not compile wasm-bindgen for non-wasm targets.

So:

• This class of problem seem like at least one reason for cargo build -p prqlc isn't really cached after cargo build #3098
• I'm not sure how to generalize it, or whether it's even viable to attempt to do that (I'm guessing it's not — new conflicts could come about at any time...)