Conditionally enable bindings for wasm32-wasi with --wasi flag

[temeddix]

With #3233(Jan 2023), generating bindings for WASI was disabled.

• Disable support for wasm32-wasi #3233

However, with #3595(Sep 2023) wasm-bindgen gained compatibility with wasm32-wasi's standard C ABI. It was stated in #3454 by @Liamolucko that wasm-bindgen already supports the standard C ABI that wasm32-wasi uses.

• Make wasm-bindgen compatible with the standard C ABI #3595
• Align with the "C" ABI #3454 (comment)

This PR aims to enable bindings for wasm32-wasi under the new --wasi flag. This will allow ecosystem around wasm-bindgen such as wasm-pack to make use of wasm32-wasi.

I'm very open to feedback, so please tag me if there's room for improvements. I should perhaps add or modify some tests for now.

[daxpedda]

Personally I have stated in #3421 why I think enabling wasm32-wasi in wasm-bindgen directly is a mistake.

So I think the way forward here is to continue discussion in #3421 to see if we can find a good reason to actually support that.

[daxpedda]

Also, please make sure it makes sense to move forward with a PR before you make one with us. It feels really bad closing PRs like that, I prefer not to waste anybodies time.