Add favicon. (#75)

index.html

@@ -8,6 +8,10 @@
   <meta name="description"
     content="Interactive playground for experimenting with Slang shading language. Write, compile, and visualize shader code in real-time." />
   <meta name="keywords" content="Slang, shader, WebGPU, graphics programming, code playground, real-time rendering" />
+  <link rel="icon" type="image/png" href="/images/favicon-96x96.png" sizes="96x96" />
+  <link rel="icon" type="image/svg+xml" href="/images/favicon.svg" />
+  <link rel="shortcut icon" href="/images/favicon.ico" />
+  <link rel="apple-touch-icon" sizes="180x180" href="/images/apple-touch-icon.png" />
   <link href="./styles/styles.css" rel="stylesheet" />
   <link href="json-tree/jsontree.js.css" rel="stylesheet" />
   <script src="https://unpkg.com/split-grid/dist/split-grid.js"></script>