Allow NonSend components

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

Allow data that is not `Send` to be stored in components, increasing API flexibility.

This is particularly useful for structs that contain a `Cell` or similar, allowing for interior (within-system) parallelization.

## What solution would you like?

Mirror / share the `NonSend` resource API. Systems that have at least one `NonSend` component or resource access can only run on the main thread to ensure that the data does not have to be passed between threads.

Reusing `NonSend` for both resources and components would be the best naming scheme for this if possible.

## What alternative(s) have you considered?

Store a reference to a `NonSend` resource maybe? Just Don't Do That.

## Additional context

This will impact the `Component` trait (#1843), as it will not include `Send + Sync` trait bounds. Instead, those will be limited to using components in a parallelizable fashion.