Web: implement WaitUntilStrategy

[daxpedda]

This implements an alternative scheduling strategy for ControlFlow::WaitUntil on Web to prevent throttling when the tab is not focused by using a worker, which commonly browsers don't throttle regardless of tab focus.

I also added another step to the CI to add JS minification, let me know if you want me to remove it.

Fixes #3738.

[madsmtm]

Hmm, I'm confused as to why this is in JavaScript? Is it not possible to write the worker in Rust? Or is it because it'd require a cumbersome build.rs step or similar to first compile the Rust code to WASM, and then embed that WASM into the final application?

(I'm totally fine with this being JS either way, mostly just curious).

[daxpedda]

Unfortunately its currently impossible to spawn a worker without at least some JS.
Even the current plan of the Shared Everything Threads Proposal requires a polyfill written in JS to make it work.

In any case, until the component model is stable and integrated a lot of functionality currently requires JS anyway, its just that it is something wasm-bindgen usually takes care of. However spawning workers unfortunately isn't one of those things.

I was briefly considering waiting until I release my web-thread crate, but realized that it would require a custom JS solution for single-threaded Wasm anyway, so this works for both.