Add `isPhoneNumber(text)` function

[Monsieur-Nico]

Create

isPhoneNumber(text: string): boolean

that returns true for common phone-number formats (international with “+” prefix, or local patterns like (123) 456-7890, 123-456-7890) and false otherwise.

Example usage:

import { isPhoneNumber } from 'textconvert';

isPhoneNumber('+1-202-555-0173');  // true
isPhoneNumber('(202) 555 0173');   // true
isPhoneNumber('5550173');          // false (too short)