replace some more `charAt` cases with bracket access #1310

Job	Run time
list-branches	9s
Build for json-source-stable	9s
Build for data-view-clamped-stage-2	6s
Build for v4	33s
Build for v3	9s
Build for v4-types	35s
Build for master	5s
Build for web	8s
Build for proposals-types	8s
combine-pages-and-deploy	15s
	2m 17s

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