obfuscate
Obfuscates this String by replacing characters at every freq-th position with '*'.
Positions count code points, so a surrogate pair (such as an emoji) is replaced or kept as a whole.
Return
the obfuscated string
Parameters
freq
the replacement frequency (default every 2nd character, starting at index 0)
Throws
if freq is not positive (it is used as a modulus divisor).