This PR adds a RawMenuAnchor() widget to widgets.dart. The purpose of this widget is to provide a menu primitive for the Material and Cupertino libraries (and others) to build upon. Additionally, this PR makes MenuController an inherited widget to simplify nested access to the menu (e.g. if you want to launch a context menu from a deeply-nested widget).

This PR:

• Centralizes core menu logic to a private class, _RawMenuAnchor(),
  • Provides the internals for interacting with menus:
    • TapRegion interop
    • DismissMenuAction handler
    • Close on scroll/resize
    • Focus traversal information, if applicable
    • Subclasses override _open, _close, _isOpen, _buildAnchor, and _menuScopeNode
  • State is accessible by descendents via MenuController.maybeOf(context)._anchor
• Adds 2 public constructors, backed by a _RawMenuAnchor() that contains shared logic.
  • RawMenuAnchor()
    • Users build the overlay from scratch.
    • Provides anchor/overlay position information and TapRegionGroupId to builder
    • Does not provide FocusScope management.
  • RawMenuAnchorGroup()
    • A primitive for menus that do not have overlays (menu bars).
    • This was previously called RawMenuAnchor.node(), but @dkwingsmt made a good case for splitting out the constructor.

Documentation examples have been added, and can be viewed at https://menu-anchor.web.app/

https://github.com/user-attachments/assets/25d35f23-2aad-4d07-9172-5c3fd65d53cf

@dkwingsmt

List which issues are fixed by this PR. #143712

Some issues that need to be addressed:

Semantics:

I'm basing the menu semantics off of the comment here, but I'm unsure whether the route should be given a name. There is no menubar/menu/menuitem role in Flutter, so I'm assuming the menu should be composed of nested dialogs

Unlike the menubar pattern from W3C, the RawMenuAnchor

While it is possible to nest menus -- for example, a dropdown anchor within a full-app context menu area -- nested menus behave as a single group. I was considering adding an additional parameter that separates nested root menus from their parents, and am interested to hear your feedback.

If you had to change anything in the flutter/tests repo, include a link to the migration guide as per the breaking change policy.

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, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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