Migrate ContextMenuButtonType to an enum-like class ContextMenuButtonKind #193527

ContextMenuButtonType is a public enum in package:flutter/widgets.dart. cupertino_ui and material_ui (and probably other packages and apps) use exhaustive switches over it, so adding a new value is a breaking change. For example, adding ContextMenuButtonType.translate breaks the published cupertino_ui 1.1.1.

This is blocking the iOS Translate context menu item (#180021). The engine side already landed in #189878, but the framework side can't add a new button type without breaking the decoupled packages.
The Dart team isn't planning to add open enums (dart-lang/sdk#63678 (comment)), so the suggestion from the discussion was to move to an enum-like class, like we did with FloatingActionButtonLocation.

Because cupertino_ui and material_ui support older stable Flutter versions, they can't reference ContextMenuButtonKind until it reaches stable and they bump their minimum SDK constraint. To avoid unresolvable deprecation warnings on master in the meantime, we're doing a phased migration:

• 1. Add ContextMenuButtonKind (without deprecating ContextMenuButtonType): Introduce ContextMenuButtonKind (including translate) and ContextMenuButtonItem.kind, while keeping ContextMenuButtonType and ContextMenuButtonItem.type undeprecated. (Add ContextMenuButtonKind #193528)
• 2. Migrate cupertino_ui and material_ui: Once ContextMenuButtonKind reaches stable, bump the minimum Flutter SDK in cupertino_ui and material_ui and switch them from item.type to item.kind (including handling translate).
• 3. Deprecate ContextMenuButtonType and ContextMenuButtonItem.type