feat: add feather brush tool with adjustable settings and blending functionality

- Implemented feather brush tool in the brushRaster module, allowing for feathered edges in brush strokes.
- Added new FeatherControls component for UI adjustments of feather settings including size, radius, strength, and smoothing.
- Updated brush preview logic to accommodate feather tool alongside existing brush and eraser tools.
- Enhanced layer rendering to support feather mask previews and interactions.
- Introduced blending logic for feathered strokes to mix blurred mask values with original pixels.
- Added unit tests for feather blending functionality and tool keybindings.
- Updated cursor handling to reflect feather tool usage.

editor/state.ts

@@ -35,6 +35,8 @@ export type BrushStrokePreviewState = {
   layerId: LayerId;
   assetId: AssetId;
   source: string;
+  pendingTargetLayerId?: LayerId;
+  intrinsicSize?: Size;
 };
 
 export type MaskShapeSession = {