chore(algebra/algebra/spectrum): split file (#19161)
This splits off all the material related to polynomials from `algebra.algebra.spectrum` into a new file `field_theory.is_alg_closed.spectrum`, because (almost) all of it requires `is_alg_closed 𝕜`. This significantly simplifies the import tree for this file, and for a few files which import it.
This also does two minor housekeeping chores:
1. generalizes type class assumptions for `alg_hom.mem_resolvent_set_apply` and `alg_hom.spectrum_apply_subset`
2. rephrases `spectrum.nonempty_of_is_alg_closed_of_finite_dimensional` to use `set.nonempty`.
