Make a separate `examples` crate that uses `bevy` for our examples

> Perhaps we should consider alternatives like a separate "examples" crate, that consumes bevy as a dependency. This would give us the freedom to compile bevy examples by default with no manual feature opt-in and a ton of "helper features".

I'm in favor of this design; I think we should do that during the 0.20 cycle and close out this PR.

_Originally posted by @alice-i-cecile in https://github.com/bevyengine/bevy/issues/25211#issuecomment-5211386411_

Start by reading issue #25211 and reviewing the repository’s current examples and crate configuration. The work is done when examples live in a separate crate that consumes bevy, compile by default, and no longer require manual feature opt-in or helper features.

Written by the indexing model from the issue text.