Add more granular features to bevy_ecs

[alice-i-cecile]

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

Other projects may want to depend on bevy_ecs, but cannot do so comfortably due to the inclusion of irrelevant code.

See DioxusLabs/taffy#130 for an example.

What solution would you like?

Add granular feature flags to bevy_ecs:

• events
• systems
• scheduling (depends on systems)

All of these would be default features.

Only core data structures (World, Entity, Query and so on) would be included without a feature flag.

What alternative(s) have you considered?

Eat the longer compile times and rely on rustc to optimize away the relevant code.

Use a lighter-weight ECS like hecs instead.

Additional context

This would make #3786 even more reasonable and feasible; this could simply be another feature flag.