Fixes#845 as discussed.
Comparing the two approaches of getting `argv`:
1. The "old" way, through `CLI::argv()`:
✔️ Works automatically and almost everywhere
❌ Small abstraction overhead on macOS
❌ Does not work in weird edge-cases such as missing `/proc`
2. This PR, through `app.ensure_utf8`:
✔️ True zero-overhead abstraction: you don't pay for what you don't use
✔️ Less moving parts than the "old" approach, probably can't be broken
❌ Requires extra code to be written by the user (which is sad because
ideally everyone should use this by default)
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Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
