added regex checking for Capfriendly

The regex was made using the given information from their "Change Username" page: https://i.imgur.com/TrrOWl5.png

sherlock/resources/data.json

@@ -286,6 +286,7 @@
     "errorMsg": "<div class=\"err show p5\">No results found</div>",
     "errorType": "message",
     "rank": 64100,
+    "regexCheck": "^[a-zA-z][a-zA-Z0-9_]{2,79}$",
     "url": "https://www.capfriendly.com/users/{}",
     "urlMain": "https://www.capfriendly.com/",
     "username_claimed": "thisactuallyexists",