Replace `which` command to find if the system has `gnome-screeshot`

[gbladislau]

I've had multiple problems using pyscreeze on systems that do not come with the which command.

Looking at the code, I see that you use it to check the gnome-screeshot dependency.

Wouldn't it be more conventional to use the actuall gnome-screenshot command to check if it exists? (like gnome-screenshot --version)
By using which you create a dependency on systems that have which by default (not the case for fedora versions on the GitHub CI for example) .