Document charCodeAt edge case behavior in first line #35575

Job	Run time
coverage	11m 32s
lint	1m 2s
knip	40s
typecheck	44s
browser-integration	1m 43s
baselines	9m 12s
self-check	57s
package-size	1m 32s
Test Node 24 on ubuntu-latest	8m 30s
format	27s
Test Node 24 on windows-latest	10m 29s
Test Node 14 on windows-latest	14m 27s
misc	30s
smoke	57s
Test Node 14 on ubuntu-latest	11m 57s
Test Node lts/* on ubuntu-latest with --no-bundle	11m 16s
required	3s
	1h 25m 58s

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