feat(gray-scott): add reaction-diffusion demo

Add a Gray-Scott page with a shader ping-pong on float textures,
five presets and pointer painting.
Tick the Gray-Scott item in ROADMAP.md.

src/app/constants/RouterConstants.ts

@@ -65,6 +65,11 @@
     LINK: '/algorithms/physarum',
   };
 
+  static readonly GRAY_SCOTT = {
+    PATH: 'algorithms/gray-scott',
+    LINK: '/algorithms/gray-scott',
+  };
+
   static readonly FOUR_COLOR = {
     PATH: 'algorithms/four_color',
     LINK: '/algorithms/four_color',