Add contextMenu

[colemancda]

.contextMenu { … } — a menu of actions revealed by long-pressing a view.

Shape

The menu items are views, so — like .sheet and .toolbar — they can't ride on a scalar modifier. The wrapper emits a ContextMenu node whose children are [content…, menuItem…], with a contentCount prop marking the split. The interpreter wraps the content in a combinedClickable(onLongClick =) and anchors a DropdownMenu of the items to it — reusing the exact item-dispatch logic from Menu, so each item is a Button that fires its own callback and dismisses.

Verification

• swift test — new test, 116 passing: the wrapper carries contentCount = 1, holds 1 content + 2 menu items, and a menu item's callback still reaches the closure through the hidden-child path.
• Emulator (new "Context Menu" screen), screenshot-checked: long-pressing the "Project Alpha" row opened a dropdown anchored to it (Rename / Duplicate / Delete), and tapping Delete fired its action (status → "Deleted Alpha") and dismissed the menu. The second example's menu reads live state (Pin ⇄ Unpin).

Scope

Buttons as menu items (the common case); no submenus, no .contextMenu(menuItems:preview:) preview variant, and no primary-action overload.