Refactor/regex10digit

[WillemZeiler]

PR Description:

This was an existing script containing a (basic) regular expression.
Updated the code and readme:

Update script.js

• Renamed variable reg to digitLengthRegex to better reflect its purpose.
• Replaced string-based regex ('/^\d{10}$/') with a proper RegExp object.
• Added test cases for strings with 9, 10, and 11 digits to demonstrate expected behavior.
• Improved inline comments.

Update README.md
Updated the readme to be more descriptive and include instructions how to adjust the code.

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[ivan-betev]

Looks good.