Proposal: Despawn entities with the `Component` macro.

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

Defining hooks can be simplified by a macro in some cases.

## What solution would you like?

Add a `#[component(despawn)]` field attribute, this field is either `Entity` or `&T: for<'t> impl IntoIterator` (including `Option`, `Vec`, etc). When the `on_remove` hook is triggered, despawn these entities.

## What alternative(s) have you considered?

Implement this manually.

## Additional context

Currently we have to pull in `bevy_hierarchy` to implement this in the macro, using `DespawnRecursive`. If we use hooks in 0.15 to maintain hierarchy this implementation can be simpler and work with `bevy_ecs` only.