Use thread_local instead of (deprecated) JsStatic

[maxime-tournier]

Hello there,

I tried compiling your crate with a recent rust nightly, which ended up in compilation errors.

I could fix them using thread_local instead of JsStatic (as per https://rustwasm.github.io/wasm-bindgen/reference/static-js-objects.html), hence this PR.

Please let me know if this seems reasonable to you?

[wngr]

Thanks!