bevy_reflect: Add `reflect_fn!` macro #32640

Job	Run time
build-and-install-on-iOS	0s
build-without-default-features	0s
build-android	0s
check-example-showcase-patches-still-work	0s
check-unused-dependencies	0s
run-examples-linux-vulkan	0s
run-examples-on-wasm	0s
run-examples-on-windows-dx12	0s
build-without-default-features-status	0s
Compare Linux screenshots	0s
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