Updates the iOS text selection toolbar to look like iOS 16 (reland)

This reverts commit 1cb16a1e.

Updates the text selection toolbar on iOS light mode to match changes in native iOS 16.

* Can show context menus anywhere in the app, not just on text.
* Unifies all desktop/mobile context menus to go through one class (ContextMenuController).
* All context menus are now just plain widgets that can be fully customized.
* Existing default context menus can be customized and reused.

Landing on red to green the build.

Fix for text selection toolbar position in cases with overflowing text.

* more const immutable classes

* more const constructors in dev/ and examples/

* Use `curly_braces_in_flow_control_structures` for `animation`

* Use `curly_braces_in_flow_control_structures` for `cupertino`

* fix comments

  * Making a custom iOS-style text selection menu is now much easier.
  * Exposes a new widget for the toolbar, a new widget for the buttons, and a new widget for the layout.

Adds MediaQuery.maybeOf to replace calling MediaQuery.of(context, nullOk: true), and removes the nullOk parameter. Also changes MediaQuery.of to return a non-nullable value, and removes many instances of the ! operator, reducing the possible places where a null dereference could occur.

The remaining uses of this pattern were all due to wanting to have
the child's slot when `Element.forgetChild()` was called. However,
when that method is called, the child's `slot` value is still valid
in the context of the parent, so the uses can just use `child.slot`.

This is the final round of cleanup from the fallout of #63269

This reverts commit ce40de69.

(this re-applies #64189 by reverting #64249)