Improve monomial_coefficients for Lie algebras when they are known to be finite dimensional

[tscrim]

Currently, monomial_coefficients() for Lie algebras plays it safe by not doing the computation when it is not known to have a distinguished basis or to be finite dimensional. Specifically, if the Lie algebra has set the _gens attribute, then it is considered unsafe (e.g., it is a LieAlgebraFromAssociative). However, in the cases it can compute basis(), which means we should be able to do monomial_coefficients().

Component: algebra

Keywords: lie algebras

Issue created by migration from https://trac.sagemath.org/ticket/25033