feat: port Probability.Process.Stopping (#5213)

Author: Parcly-Taxel

Mathlib.lean

@@ -2448,6 +2448,7 @@ import Mathlib.Probability.ProbabilityMassFunction.Monad
 import Mathlib.Probability.ProbabilityMassFunction.Uniform
 import Mathlib.Probability.Process.Adapted
 import Mathlib.Probability.Process.Filtration
+import Mathlib.Probability.Process.Stopping
 import Mathlib.RepresentationTheory.Action
 import Mathlib.RepresentationTheory.Basic
 import Mathlib.RepresentationTheory.Maschke