[poincare] Fix VerticalOffsetLayout serialization

"2^3 !" would be serialized as "2^3*!", which is wrong. It is now serialized
as "2 Ion::Charset::Superscript 3 Ion::Charset::Superscript !" and
parsed correctly.

ion/include/ion/charset.h

@@ -27,7 +27,8 @@ enum Charset : char {
   MiddleDot = (char)149,
   AlmostEqual = (char)150,
   Degree = (char)151,
-  Empty = (char)152 // This char is used to be parsed into EmptyExpression.
+  Empty = (char)152, // This char is used to be parsed into EmptyExpression
+  Superscript = (char)153 // This char is used to parse Power
 };
 
 }