path: remove `StringPrototypeCharCodeAt` from some methods of `posix` #1863

Job	Run time
Build slim tarball	1m 39s
x86_64-darwin: with shared libraries	46m 6s
aarch64-darwin: with shared libraries	25m 7s
x86_64-linux: with shared libraries	28m 41s
aarch64-linux: with shared libraries	24m 20s
	2h 5m 53s

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