Lift Send requirement in wasm builds

[extrawurst]

What problem does this solve or what need does it fill?

Right now there is a requirement for a lot of things to be Send even in single-threaded targets like wasm: e.g Plugin and Resource. Would be great to lift that in wasm targets.

What solution would you like?

Do not require things to be Send

What alternative(s) have you considered?

Additional context

This blocks being able to use bevy_tokio_tasks for more stuff on wasm: see EkardNT/bevy-tokio-tasks#18 (comment)

[konsti219]

Wasm is not single-threaded. You can create multi-threaded wasm applications that run in browsers today.

Afaik bevy does not currently make use of this but keeping it as a future option without a breaking change is the way I personally would go.

[james7132]

Please see #4078 for more information. We eventually want to support full use of Web Workers in all WASM targets, but there are fundamental limitations that we may need to find a workaround for. Knowing this, conditionally lifting Send constraints, where relevant, seems shortsighted.

[james7132]

With the work in #12205, it's pretty apparent that any !Send component or resource on wasm can be wrapped in send_wrapper or bevy_utils::SyncCell to achieve Send + Sync required for Resources or Components. If the actual environment is truly single threaded, both should be a non-issue to deal with. Closing this out for now.