Remove wasm feature

[twifkak]

In Rust, features are meant to be additive -- only introducing new APIs, not modifying or removing existing ones. Our wasm feature modifies existing APIs, leading to issues (e.g. bare cargo test fails because some packages require sxg_rs with wasm and others are incompatible with wasm).

Instead, I think we should make our wasm code conditional on target_family="wasm", and update our .github/workflow configs to build/test for both architectures, Linux and WASM (see rustc --print target-list for options and rustc -vV to print the default target).

I'm not sure how to disable certain packages on the wasm architecture, though, so maybe it doesn't solve the cargo test issue in particular.