Make open subsets of immersed/embedded submanifolds immersed/embedded submanifolds #31674
Description
This would be convenient for example for #31660.
To implement this, we refactor the method open_subset, already now duplicated between TopologicalManifold and DifferentiableManifold through a new method _init_open_subset.
CC: @mjungmath @egourgoulhon @tscrim
Component: manifolds
Author: Matthias Koeppe
Branch/Commit: 58b43f7
Reviewer: Eric Gourgoulhon
Issue created by migration from https://trac.sagemath.org/ticket/31674