chore: Split HahnSeries file (#10595)

The HahnSeries file is approaching 2000 lines, and I was hoping to add some more material on Hahn series in the near future.  This PR splits the material roughly according to what structure we see in the coefficients: `Basic` assumes that the coefficient type `R` has zero, `Addition` assumes addition, `Multiplication` more or less assumes some multiplicative structure.

Author: ScottCarnahan

Mathlib.lean

@@ -3211,7 +3211,11 @@ import Mathlib.RingTheory.GradedAlgebra.Basic
 import Mathlib.RingTheory.GradedAlgebra.HomogeneousIdeal
 import Mathlib.RingTheory.GradedAlgebra.HomogeneousLocalization
 import Mathlib.RingTheory.GradedAlgebra.Radical
-import Mathlib.RingTheory.HahnSeries
+import Mathlib.RingTheory.HahnSeries.Addition
+import Mathlib.RingTheory.HahnSeries.Basic
+import Mathlib.RingTheory.HahnSeries.Multiplication
+import Mathlib.RingTheory.HahnSeries.PowerSeries
+import Mathlib.RingTheory.HahnSeries.Summable
 import Mathlib.RingTheory.Henselian
 import Mathlib.RingTheory.Ideal.AssociatedPrime
 import Mathlib.RingTheory.Ideal.Basic