gh-38672: src/sage/features/giac.py: add new feature for the giac program
In preparation for adding a `--disable-giac` option, we add a new
feature that detects the presence of the "giac" executable. We already
have a feature for `sage.libs.giac`, but that only guards the libgiac
interface; we still have code that runs "giac" behind pexpect. This will
allow us to skip those tests when giac is not installed.
URL: https://github.com/sagemath/sage/pull/38672
Reported by: Michael Orlitzky
Reviewer(s): Dima Pasechnik
