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.

view/paletteItems.tsx

@@ -1,4 +1,4 @@
-import { ArrowCounterClockwise, ArrowDown, ArrowUp, CornersOut, Cursor, DownloadSimple, DropHalf, Eraser, Eye, EyeSlash, FolderOpen, FolderPlus, Hand, Lock, LockOpen, MagicWand, Minus, PaintBrush, Plus, Sparkle, Stack, Trash } from "@phosphor-icons/react";
+import { ArrowCounterClockwise, ArrowDown, ArrowUp, CornersOut, Cursor, DownloadSimple, DropHalf, Eraser, Eye, EyeSlash, Feather, FolderOpen, FolderPlus, Hand, Lock, LockOpen, MagicWand, Minus, PaintBrush, Plus, Sparkle, Stack, Trash } from "@phosphor-icons/react";
 import type { ReactNode } from "react";
 import { commandIds } from "@commands/ids";
 import type { Artboard } from "@core/artboard";
@@ -343,6 +343,8 @@ function toolIcon(tool: ToolId) {
       return <PaintBrush size={20} />;
     case "eraser":
       return <Eraser size={20} />;
+    case "feather":
+      return <Feather size={20} />;
     case "magicWand":
       return <MagicWand size={20} />;
     case "semanticSelect":