extract substring tool, testCases then updated index file

src/pages/string/extract-substring/index.tsx

@@ -0,0 +1,11 @@
+import { Box } from '@mui/material';
+import React from 'react';
+import * as Yup from 'yup';
+
+const initialValues = {};
+const validationSchema = Yup.object({
+  // splitSeparator: Yup.string().required('The separator is required')
+});
+export default function ExtractSubstring() {
+  return <Box>Lorem ipsum</Box>;
+}