code point to check
true if and only if c is a valid Unicode code point
Note: '\uFFFE' and '\uFFFF' are considered valid by isValidDchar, as they are permitted for internal use by an application, but they are not allowed for interchange by the Unicode standard.
assert( isValidDchar(cast(dchar) 0x41)); assert( isValidDchar(cast(dchar) 0x00)); assert(!isValidDchar(cast(dchar) 0xD800)); assert(!isValidDchar(cast(dchar) 0x11FFFF));
Check whether the given Unicode code point is valid.