add `iter()` method for StringView #4820

Job	Run time
bleeding-check (macos-latest)	1m 18s
build (ubuntu-latest)	2m 36s
coverage-check	15s
license-header-check	4s
build (macos-latest)	1m 48s
bleeding-check (ubuntu-latest)	2m 1s
build (macos-13)	2m 47s
build (windows-latest)	2m 3s
moon-json-format-check	3s
nightly-check (ubuntu-latest)	2m 2s
typo-check	6s
nightly-check (macos-latest)	1m 18s
nightly-check (macos-13)	2m 16s
nightly-check (windows-latest)	1m 19s
	19m 56s

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