Debug tool to validate archetypes

[tangmi]

If a user is unfamiliar with a plugin, it's currently difficult to know if an entity has the right components for the plugin's systems. Currently one must rely on docs or looking at the plugin's implementation to know for sure.

I'd like to propose a tool that could provide optional warnings to the user in some scenarios (there's likely many more helpful cases I've missed):

• When a system is registered, but unused. This could help users ensure the systems they want are added and potentially help identify unneeded bloat.
• When an entity* that has unused components.
  • ... and almost meets the requirements for a system. This would help identify unnecessary components, but could be a very useful help to say "Hey, I see you've added x component and y system, but y system also needs x' component".

This could come in the form in optional (or debug-only) warning messages that go through the logger or a set of APIs (that could e.g. be consumed by the eventual bevy editor UI).

Note that this idea isn't fully formed and mostly comes out of my own observations while using bevy. I'd like to use this discussion to further explore the UX regarding archetype/entity/system debug tools.

*This was briefly discussed on Discord and @cart suggested doing some of the validation at the archetype level as an optimization.