Add `which`

[tats-u]

• where: Where-Object in PowerShell
• Get-Command/gcm: You have to use (gcm COMMAND).path to get the path string, which is too verbose

Ubuntu package: debianutils (https://salsa.debian.org/debian/debianutils) LICENSE
RHEL package: which (https://savannah.gnu.org/projects/which/)
GNUWin32: https://gnuwin32.sourceforge.net/packages/which.htm

Unfortunately all of them are licensed under GPL. No MIT alternatives, even in https://github.com/uutils/.

[caomengxuan666]

See #59

[tats-u]

You can link this issue to your PR.

[lhecker]

Dupe of #59.

[lhecker]

Ah, didn't see that it was a PR.