Add Read-only system::run_read_only

[alice-i-cecile]

          Good call! As a potential followup (idk if we actually need this) we could add a `ReadOnlySystem::run_readonly()`  that allows you to safely run the system with an `&World`.

Originally posted by @cart in #7547 (review)

I think this would be valuable, and should be a very straightforward method to implement. Look at how System's method works for reference.

[joseph-gio]

I think we should wait until we're done with properly migrating the engine to UnsafeWorldCell. &World is still being used for interior mutability, so it feels a bit odd to also use &World to guarantee immutable access.

[joseph-gio]

This is unblocked by #8833.