feat: port Algebra.Lie.Nilpotent (#4919)

Author: Ruben-VandeVelde

Mathlib.lean

@@ -217,6 +217,7 @@ import Mathlib.Algebra.Lie.DirectSum
 import Mathlib.Algebra.Lie.Free
 import Mathlib.Algebra.Lie.IdealOperations
 import Mathlib.Algebra.Lie.Matrix
+import Mathlib.Algebra.Lie.Nilpotent
 import Mathlib.Algebra.Lie.NonUnitalNonAssocAlgebra
 import Mathlib.Algebra.Lie.Normalizer
 import Mathlib.Algebra.Lie.OfAssociative