Replace setup_web.sh with something more robust

[jleibs]

setup_web.sh is currently used to install both wasm-bindgen-cli and binaryen.

It has a few problems that keeps it from being portable:

• depends on a bash shell
• windows doesn't have a package manager in CI
• The installed binaryen isn't versioned and the version installed on 20.04 is too old
• The script will still fail even if the packages were manually installed correctly.

The basic logic should be:

• Check that the packages are installed with the correct version. If yes, always return as a no-op.
• If no, try to install with appropriate system package manager target.
• If that doesn't work, link to instructions for manual installation and fail.

[jleibs]

This is now duplicated by the other pixi-related work.