Add helper dfrapi_string_starts_with() function

[EricBusch]

Add the following function:

/**
 * Returns true if the $string starts with one of the $patterns. Otherwise returns false.
 *
 * @param string $string The haystack.
 * @param string|array $patterns The patterns to search for in the beginning of the $string.
 *
 * @return bool
 */
function dfrapi_string_starts_with( $string, $patterns ) {
	$patterns = ( is_string( $patterns ) ) ? array( $patterns ) : $patterns;
	foreach ( $patterns as $pattern ) {
		$length = mb_strlen( $pattern );
		if ( mb_substr( $string, 0, $length ) === $pattern ) {
			return true;
		}
	}

	return false;
}