gh-34970: add is_supergreedy() to linear extensions
This is my pull request originally created on trac, which I thought
pushing on github.
Fixes #24700
URL: https://github.com/sagemath/sage/pull/34970
Reported by: Rohan Garg
Reviewer(s): Dima Pasechnik, Martin Rubey, Rohan Garg, Tobias Diez
