Replace run-wasm-example.sh with aliased rust crate (xtask)

[rukai]

To use the new method simply do:
cargo run-wasm --example cube --features webgl

Advantages over existing solution

• cross platform
• 0 external dependencies - the existing solution depends on bash, a webserver and wasm_bindgen.
• better UX + more robust than anything hacked together with bash/powershell/bat
• wasm-bindgen-cli version is always in sync with wasm-bindgen version because cargo update updates both of them at the same time thanks to being in the same workspace

Currently uses a git dependency, but i'll release to crates.io when we are happy with the API.

Testing
I ran cargo run-wasm --example cube --features webgl etc for every example on linux.
I confirmed at least the cube example works on mac os and windows.

The documentation at https://github.com/gfx-rs/wgpu/wiki/Running-on-the-Web-with-WebGPU-and-WebGL will need to be changed after merging.

There is a PR in progress to use this same crate for running winit's web examples rust-windowing/winit#2115

[kvark]

please rebase

[rukai]

I just noticed a potential problem with using wasm-bindgen-cli-support: rustwasm/wasm-bindgen#2757
If it turns out to be a problem I'll figure out some solution to avoid cargo-run-wasm constantly breaking.