glTF labels: add enum to avoid misspelling and keep up-to-date list documented #43067

Job	Run time
ci	2m 0s
check-doc	8m 8s
build (windows-latest)	4m 23s
check-missing-examples-in-docs	31s
build (ubuntu-latest)	2m 22s
miri	5m 40s
build (macos-latest)	1m 44s
run-examples-macos-metal	4m 53s
toml	1m 50s
typos	5s
check-bevy-internal-imports	5s
check-cfg	1m 41s
check-missing-features-in-docs	41s
markdownlint	1m 17s
check-compiles	3m 58s
build-wasm	38s
build-wasm-atomics	1m 27s
msrv	54s
	42m 17s

Provide feedback