Document charCodeAt edge case behavior in first line #35573

Job	Run time
coverage	11m 32s
lint	59s
Test Node lts/* on ubuntu-latest with --no-bundle	10m 37s
smoke	55s
typecheck	43s
Test Node 24 on windows-latest	10m 39s
Test Node 14 on ubuntu-latest	12m 4s
format	24s
Test Node 24 on ubuntu-latest	8m 37s
Test Node 14 on windows-latest	14m 19s
self-check	1m 0s
browser-integration	1m 42s
package-size	1m 34s
misc	26s
knip	36s
baselines	9m 48s
required	4s
	1h 25m 59s

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