Add Modal components support

[pierpo]

Is your feature request related to a problem? Please describe.

Right now we have no clear way to have little modals on top of our screens.
There are ways to do it, but we have no clear recommendation yet 😊

This might need some additions in the core (even though I think what we have might be enough already).

Describe the solution you'd like

<SpatialNavigationRoot>
  <Button /> {/* should not be active anymore */}
  <Modal isOpen>
    <Button /> {/* should be active */}
  </Modal>
</SpatialNavigationRoot>

Describe alternatives you've considered
...

Additional context
N/A

[pierpo]

This was merged. Nothing in the core of the lib, but an example is now provided 😊

#59