(split out from #31982)

We change the function sage.modules.free_module.element_class so that it uses the element class Vector_symbolic_dense not only for free modules over the ring SR but also for free modules over subrings of SR such as the symbolic constants ring.

In fact, we dispatch through a new method _free_module_element_class_dense.

CC: @egourgoulhon @mjungmath @tscrim

Component: symbolics

Author: Matthias Koeppe

Branch/Commit: u/mkoeppe/use_vector_symbolic_dense_for_all_symbolic_rings__not_just_sr @ 52396f4

Issue created by migration from https://trac.sagemath.org/ticket/31999