Deprecate panicking .resource methods

[alice-i-cecile]

Objective

This PR is a sibling to #18082, and almost all of the motivation there applies here. These methods are much more commonly used in commands, tests and exclusive systems though, and deserve their own judgement call.

Ultimately part of #14275.

Solution

1. Deprecate all of the panicking resource methods I can find.

I'm less confident about this PR, and there's 796 warnings in my IDE, so I want to get consensus before burning time on migrating all of these.

TODO:

• fix deprecation warnings
• add docs?
• [] make World::resource and friends return a Result for nicer ? ergonomics?

Future Work

Move the non-panicking versions to the better names during the 0.17 cycle, just like in #18082.

Migration Guide

World::resource and all related methods are now deprecated. Use World::get_resource and so on and handle the returned Option.

[alice-i-cecile]

We could choose to use a lint for these methods and keep them around (see TheBevyFlock/bevy_cli#269) and keep them around, but I prefer the solution here because:

1. Our defaults should be the right choice for finished games.
2. Less API = More Good.
3. Relying on external unshipped tooling sucks.

[JMS55]

This is going to make a lot of rendering code worse :/ (check how often .resource::< is called). Why not keep both methods around? If we want to make the engine itself more resilient, then I think targeted audits to find places we shouldn't be panicking is a better idea.

[alice-i-cecile]

Yeah, I think that's reasonable. Given that we're not running into many panics here (and Bevy already panics when you're missing a resource in a normal system) I think this is net-negative. Closing out.