Add "resource_exists_and" (#23259)

[Satellile]

Adds feature: SystemCondition-satisfying closure resource_exists_and.
#23259

Testing

• I've tested this in my own game on Bevy 0.17, with some simple cases. Haven't noticed any issues.

[mockersf]

The doc example is a bit hard to read, with a counter being negative and never incremented, and not really showing the system didn't run case: it's present in the example, but not being asserted that it doesn't do anything.

I think I would prefer an example with two resources:

• a counter just being incremented by the system
• a resource ShouldRun(bool)

then:

• add the `Counter(0) resource, run, assert the counter is still 0
• add the ShouldRun(true) resource, run, assert the counter is 1
• change the resource to ShouldRun(false), run, assert the counter is still 1

[chescock]

I believe this can already be done today using If<Res>. If will cause a system to skip instead of failing if the resource does not exist, and skipping a run condition will skip its system.

// Instead of
my_system.run_if(resource_exists_and(|counter: &Counter| counter.0.is_negative()))
// I think you can use
my_system.run_if(|counter: If<Res<Counter>>| counter.0.is_negative())

[alice-i-cecile]

Sure. It's a bit niche, but given that the siblings exist this is fine.

[mockersf]

doc example should be improved

[Satellile]

Just updated the docs as recommended. However I changed ShouldRun(bool) to ShouldRun(String) and had the run condition be .is_ascii(), as I felt the example should be something that couldn't also be done by resource_exists_and_equals.