orbea/exists: Posix shell function for finding executable files in the $PATH

orbea 75426af2f1 test.sh: Test if command exits with the first missing arg.		4 anni fa
..
command-args-test.sh	8253825287 test.sh: Test if command supports multiple arguments with -v.	4 anni fa
command-args2-test.sh	75426af2f1 test.sh: Test if command exits with the first missing arg.	4 anni fa
exists.hsh	450b259d05 Init exists test repo.	5 anni fa
path-empty-test.sh	499ad79175 exists.sh: Handle when the PATH is unset correctly.	4 anni fa
printf	ec91627adc scripts/printf: Typo fix.	5 anni fa
pwd-test.sh	508406ee4e exists.sh: Return absolute paths.	4 anni fa