Emoji using ZWJ are considered invalid

[DysphoricUnicorn]

The check if a character is printable currently considers the zero width joiner unprintable, even if it is between two characters that turn into a new one with it.

This means that a lot of newer emoji are not supported.
Examples are pride flags, newer family combinations and skin tone variations.

[binaryDiv]

Just some thoughts about this:

• Simply allowing the ZWJ anywhere in a string could lead to potential problems. Imagine a string with a ZWJ at the beginning or end without any other character to connect to, which (I believe?) would be an invalid usage of the ZWJ. So a more complex and careful validation definitely is necessary.
• The StringValidator should get an option to specify the charset. In many cases emoji (in general, not just with ZWJ) might not be desirable in a string, e.g. because it doesn't make sense to use Emoji in the particular input field, or because the underlying database uses a charset that doesn't support emoji. Also, often an ASCII-only StringValidator might be needed.
• For convenience, subclasses for StringValidators with preconfigured charsets might be useful (e.g. AsciiStringValidator?).