Pull `bevy_ecs::intern` out into its own separate crate

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

The label infrastructure that `bevy_ecs` provides is not actually dependent on anything ECS-related, and could find use as a separate crate from `bevy_ecs`.

## What solution would you like?

Introduce a `bevy_label` crate that holds everything currently defined in `bevy_ecs::label`.

## What alternative(s) have you considered?

Leave it as-is.