Check for conflicting accesses in assert_is_system

[JoJoJet]

Objective

The function assert_is_system is used in documentation tests to ensure that example code actually produces valid systems. Currently, assert_is_system just checks that each function parameter implements SystemParam. To further check the validity of the system, we should initialize the passed system so that it will be checked for conflicting accesses. Not only does this enforce the validity of our examples, but it provides a convenient way to demonstrate conflicting accesses via a should_panic example, which is nicely rendered by rustdoc:

Solution

Initialize the system with an empty world to trigger its internal access conflict checks.

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Changelog

The function bevy::ecs::system::assert_is_system now panics when passed a system with conflicting world accesses, as does assert_is_read_only_system.

Migration Guide

The functions assert_is_system and assert_is_read_only_system (in bevy_ecs::system) now panic if the passed system has invalid world accesses. Any tests that called this function on a system with invalid accesses will now fail. Either fix the system's conflicting accesses, or specify that the test is meant to fail:

1. For regular tests (that is, functions annotated with #[test]), add the #[should_panic] attribute to the function.
2. For documentation tests, add should_panic to the start of the code block: ```should_panic

[alice-i-cecile]

Good docs, and good functionality.

[Pietrek14]

LGTM

[JoJoJet]

Thank you for the reviews. I have filled in the migration guide.