Add ArgumentParser::mIsParsed to copied members

MSVC 19.16 appears to be doing a copy rather than a move in
test_const_correct.  The copy ctor does not handle mIsParsed, so the
initial false value is kept.  This commit adds copying mIsParsed during
copy construction.

Signed-off-by: Sean Robinson <sean.robinson@scottsdalecc.edu>

include/argparse/argparse.hpp

@@ -830,6 +830,7 @@ public:
 
   ArgumentParser(const ArgumentParser &other)
       : mProgramName(other.mProgramName),
+        mIsParsed(other.mIsParsed),
         mPositionalArguments(other.mPositionalArguments),
         mOptionalArguments(other.mOptionalArguments) {
     for (auto it = std::begin(mPositionalArguments); it != std::end(mPositionalArguments);