Add support for multi-window popups on Windows

[hbatagelo]

Design doc: https://docs.google.com/document/d/1eQG-IS7r4_S9_h50MY_hSGwUVtgSTRSLzu7MLzPNd2Y/edit?tab=t.0

This PR is a part of a group of PRs. Going from first to last, these must be merged in the following order:

• Add support for multiple top-level windows on Windows #56090
• Add support for multi-window popups on Windows #56092
• Add support for multi-window dialogs on Windows #56093
• Add support for multi-window satellites on Windows #56094

See also the corresponding framework PRs for multi-win support, starting from: flutter/flutter#157515

What's New

This pull request adds support for multi-window popups in the Windows embedder and introduces logic for positioning child windows relative to a parent view or parent window.

A new method is available in the flutter/windowing channel:

• createPopup(Map)
Map must contain the following key-value pairs:

  Key (String)	Value
  "parent"	The ID of the parent view (int)
  "size"	The width and height of the popup, in logical coordinates (List<int>)
  "anchorRect"	An anchor rectangle for positioning the child window (the popup) relative to the parent view. The rectangle values (left, top, width, height) are in logical coordinates, relative to the parent view (i.e., relative to the client rectangle of the parent window). If null, the child window will be positioned relative to the window frame of the parent (List<int>)
  "positionerParentAnchor"	The anchor point in the anchor rectangle (WindowPositionerAnchor)
  "positionerChildAnchor"	The anchor point in the window frame of the child window (WindowPositionerAnchor)
  "positionerOffset"	The positioning offset as x and y displacements from the parent anchor to the child anchor (List<int>)
  "positionerConstraintAdjustment"	A bitwise combination of WindowPositioner::ConstraintAdjustment, defining the adjustments to apply if the child window doesn't fit in the available space. The order of precedence is: 1) Flip, 2) Slide, 3) Resize (int)

  On success, a Map is returned with the following key-value pairs:

  Key (String)	Value
  "viewId"	The view ID of the window (int)
  "archetype"	WindowArchetype::popup
  "size"	The current size of the window (width, height), in logical coordinates (List<int>)
  "parentViewId"	The view ID of the parent window (int)

  If the method fails, a PlatformException is thrown.

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide and the C++, Objective-C, Java style guides.
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or the PR is test-exempt. See testing the engine for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with ///).
• I signed the CLA.
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.

[flutter-dashboard]

This pull request executed golden file tests, but it has not been updated in a while (20+ days). Test results from Gold expire after as many days, so this pull request will need to be updated with a fresh commit in order to get results from Gold.