Document charAt edge case behavior #35595

Job	Run time
format	24s
baselines	9m 47s
smoke	54s
package-size	1m 30s
typecheck	45s
coverage	11m 41s
knip	28s
self-check	53s
lint	59s
browser-integration	1m 37s
misc	26s
Test Node lts/* on ubuntu-latest with --no-bundle	11m 7s
Test Node 24 on windows-latest	10m 13s
Test Node 14 on ubuntu-latest	12m 29s
Test Node 24 on ubuntu-latest	8m 46s
Test Node 14 on windows-latest	14m 13s
required	2s
	1h 26m 14s

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