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

Title. This crate is basically a utilities crate, which we would like to avoid (bevy_utils is slowly being split up, for example). See here for its current API surface: https://docs.rs/bevy_core/latest/bevy_core/

What solution would you like?

We should:

• Move FrameCount and its plugin to bevy_diagnostics or bevy_window
• Move other plugins to bevy_app
• Move everything else to bevy_ecs

What alternative(s) have you considered?

Leave it as-is.

Additional context

Related to #11478 and #16877.

Originally brought up on the Bevy Discord.