echo: add support for `POSIXLY_CORRECT` #5562

Job	Run time
Style/format (ubuntu-latest, feat_os_unix)	12s
Style/lint (ubuntu-latest, feat_os_unix)	1m 23s
Style/spelling (ubuntu-latest, feat_os_unix)	40s
Style/lint (macos-latest, feat_os_macos)	1m 7s
Style/toml	8s
Style/lint (windows-latest, feat_os_windows)	2m 27s
	5m 57s

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