feat(masks): add mask editing mode

editor/initial-state.ts

@@ -13,6 +13,7 @@ export const initialEditorState: EditorState = {
   },
   tools: initialToolState,
   transformSession: undefined,
+  maskEditLayerId: undefined,
 };
 
 export function createInitialAppState(name = "Untitled"): AppState {

editor/state.ts

@@ -21,6 +21,7 @@ export type EditorState = {
   selection: SelectionState;
   tools: ToolState;
   transformSession?: TransformSession;
+  maskEditLayerId?: LayerId;
 };
 
 export type HistorySnapshot = {