feat: define C^n submersions

Mathlib.lean

@@ -4579,6 +4579,7 @@ public import Mathlib.Geometry.Manifold.Sheaf.Smooth
 public import Mathlib.Geometry.Manifold.SmoothApprox
 public import Mathlib.Geometry.Manifold.SmoothEmbedding
 public import Mathlib.Geometry.Manifold.StructureGroupoid
+public import Mathlib.Geometry.Manifold.Submersion
 public import Mathlib.Geometry.Manifold.VectorBundle.Basic
 public import Mathlib.Geometry.Manifold.VectorBundle.CovariantDerivative.Basic
 public import Mathlib.Geometry.Manifold.VectorBundle.CovariantDerivative.Torsion