Add grep exercise in config file

Author: TomPradat

config.json

@@ -1430,6 +1430,18 @@
         "strings"
       ],
       "deprecated": true
+    },
+    {
+      "slug": "grep",
+      "uuid": "78bcbae1-a0f2-460c-ba89-e51844fe9397",
+      "core": false,
+      "unlocked_by": null,
+      "difficulty": 4,
+      "topics": [
+        "files",
+        "searching",
+        "text_formatting"
+      ]
     }
   ]
 }