Support wasm splitting

Leptos recently added wasm splitting to its toolchain via islands and following the prototype described in https://github.com/wasm-bindgen/wasm-bindgen/issues/3939. It seems that `wasm-bindgen` does not, at the moment, consider adding this to their toolchain itself. I'm seemingly not able to hack this in by myself (outside a code change to trunk), due to the hooks in trunk running either too early or too late; The wasm should be split before wasm-bindgen processes it (as that seemingly throws away required relocation information), and produces multiple output artifacts for trunk (that should then, each, be further processed by `wasm-opt` and other tools as configured). I would try to hack this together into the trunk pipeline, but let me know if this would be something you consider to have first.