Rename `num_font_atlases` to `len`. #30056

Job	Run time
build (windows-latest)	5m 31s
ci	2m 59s
build (ubuntu-latest)	3m 32s
build (macos-latest)	4m 0s
check-bevy-internal-imports	6s
check-doc	7m 50s
check-missing-examples-in-docs	56s
miri	9m 6s
run-examples-on-windows-dx12	16m 52s
check-missing-features-in-docs	1m 12s
markdownlint	1m 22s
check-compiles	3m 22s
msrv	1m 20s
build-wasm	1m 2s
	59m 10s

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