Add IsMultiline(string) extension method

Add IsMultiline(string) extension method for string.

Following the spirit of #43, should probably also implement extensions for char[], Span<char>, and ReadOnlySpan<char>.

Reference Implementation

        public static bool IsMultiline(this string self)
        {
            if (self is null) { throw new NullReferenceException(); }
            if (self.Length == 0) { return false; }
            for (int index = 0; index < self.Length; index++)
            { if (self[index] is '\r' or '\n') { return true; } }
            return false;
        }