bevy_reflect: Add `Reflectable` trait #48300

Job	Run time
build (windows-latest)	6m 17s
miri	11m 53s
build (ubuntu-latest)	2m 33s
check-doc	8m 13s
ci	3m 0s
build (macos-latest)	2m 8s
run-examples-macos-metal	5m 37s
toml	1m 55s
typos	7s
check-bevy-internal-imports	5s
check-missing-examples-in-docs	29s
check-missing-features-in-docs	43s
markdownlint	1m 2s
check-compiles	4m 30s
build-wasm	48s
build-wasm-atomics	1m 35s
msrv	1m 17s
	52m 12s

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