echo: handle multibyte escape sequences (#6803)

* echo: handle multibyte escape sequences

Bug was reported, with root cause analysis, by kkew3
Added tests were derived from test cases provided by kkew3
See #6741

* Use concrete type

* Fix MSRV issue

* Fix non-UTF-8 argument handling

* Fix MSRV issue

* Fix Clippy violation

* Fix compiler warning

* Address PR comments

* Add MSRV TODO comments

* echo: use stdout_only_bytes instead of stdout_is_bytes

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Co-authored-by: Daniel Hofstetter <daniel.hofstetter@42dh.com>

Author: andrewliebenow