feat(data/nat/basic): add decidable_exists_lt deciding existence of a natural below a bound satisfying a predicate (#5864)
feat(data/nat/basic): add decidable_exists_lt deciding existence of a natural below a bound satisfying a predicate (#5864)

Given a decidable predicate `P` on naturals it is decidable if there is a natural below any bound satisying the `P`.

closes #5755





leanprover-community/mathlib,leanprover-community/mathlib