extract substring tool, testCases then updated index file

src/pages/string/extract-substring/meta.ts

@@ -0,0 +1,13 @@
+import { defineTool } from '@tools/defineTool';
+import { lazy } from 'react';
+// import image from '@assets/text.png';
+
+export const tool = defineTool('string', {
+  name: 'Extract substring',
+  path: 'extract-substring',
+  // image,
+  description: '',
+  shortDescription: '',
+  keywords: ['extract', 'substring'],
+  component: lazy(() => import('./index'))
+});