Editor commands API / effects (#1778)

This PR shrinks the commands API surface and adds a manager
(`CleanupManager`) for side effects.

### Change Type

- [x] `major` — Breaking change

### Test Plan

Use the app! Especially undo and redo. Our tests are passing but I've
found more cases where our coverage fails to catch issues.

### Release Notes

- tbd

packages/editor/src/lib/hooks/useCanvasEvents.ts

@@ -84,12 +84,14 @@ export function useCanvasEvents() {
 
 				const files = Array.from(e.dataTransfer.files)
 
+				// the drop point should be offset by the position of the editor's container
 				const rect = editor.getContainer().getBoundingClientRect()
+				const point = editor.screenToPage({ x: e.clientX - rect.x, y: e.clientY - rect.y })
 
 				await editor.putExternalContent({
 					type: 'files',
 					files,
-					point: editor.screenToPage(e.clientX - rect.x, e.clientY - rect.y),
+					point,
 					ignoreParent: false,
 				})
 			}