Issue 11620 - Emit enum member values to json output.

https://issues.dlang.org/show_bug.cgi?id=11620

Btw, please tell me if I have to update `EnumMember::syntaxCopy` as well. It is super-confusing when you have a member function which should not take any parameters, but this one does. It takes a generic Dsymbol, it even accepts NULL, and has not documented why it takes this parameter.