feat: add isUsername validator and fix empty string bug in isNumeric

[Rughena]

feat: add isUsername validator and fix empty string bug in isNumeric

Description
This Pull Request introduces a new string validator and resolves an edge case issue in an existing validator.

1. New Feature (isUsername): Created a self contained string validator in src/lib/isUsername.js and registered it alphabetically inside src/index.js. It ensures a username string has a length between 3 and 15 characters and does not contain restricted special characters (@, #, $).
2. Bug Fix (isNumeric): Added an explicit guard clause to return false if the input string is empty (str.length === 0), avoiding unintended behavior down the line.

Checklist

• PR contains only changes related; no stray files, etc.
• README updated (where applicable)
• Tests written (where applicable)
• References provided in PR (where applicable)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (a79ff98) to head (308220e).

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2883   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          114       115    +1     
  Lines         2599      2613   +14     
  Branches       658       662    +4     
=========================================
+ Hits          2599      2613   +14     

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