Add Viewport methods to find Control nodes and subwindows at global position

[HuntJSparra]

This is an alternative implementation of #86343 based on this comment suggesting that a new Viewport::gui_find_node_at_position method be created that mimics the behavior of Viewport::gui_find_control but returns any blocking subwindows at the position. For easier type handling, this PR splits that suggested method into gui_get_control_at_position for Control nodes and gui_get_window_at_position for embedded subwindows.

@cdoise-vbg I don't want to step anyone's toes, so I'm happy to close this if you're still working on that PR.

Use Case: Mimicking/simulating mouse inputs (e.g., a local multiplayer game where each user has their own cursor); creating alternative ways to interact with UI (e.g., throwing a box at a screen).

Partially Adresses: GIP#8408 (allows finding one but not all Control nodes at a point).

New Methods:
Viewport.gui_get_control_at_position(position, ignore_subwindows): Gets the Control node at the position, optionally including if the node is within an embedded subwindow.

Viewport.gui_get_window_at_position(position): Gets the embedded subwindow at the position.

Outstanding Questions:

• Should the methods be renamed to indicate they mimic the mouse behavior (i.e., their behavior is affected by Control.mouse_filter)?
• Should Viewport::gui_find_control should be renamed to Viewport::_gui_find_control, assuming we never intend for developers to use it as a public API? NOTE: I believe this would break any custom modules using gui_find_control (of which I've seen at least one).
• Some Control nodes will not behavior as expected. PopupMenu in particular is weird due to how it handles input and appears to to not actually use Buttons under-the-hood. Is this fine?