isEmail function
Checks if the string is a valid email address.
Uses a regular expression to verify if the string matches standard email format. Validates both local part (before @) and domain part (after @) of the email.
Returns true if the string is a valid email address, otherwise returns false.
Example:
isEmail('user@example.com'); // true
isEmail('user.name+tag@example.co.uk'); // true
isEmail('invalid@email'); // false
Implementation
bool isEmail(String str) => _isEmail(str);