Improve support for "const" var

qqmljs.g seems to respect T_CONST, and the IR is made aware of the information, but it seems to end there. We should probably propagate the information further from there, and actually support constant properties.

It also seems strange to me that "const" is only respected in QML mode. With the advent of ES6, perhaps we should allow it in JS syntax, too?

On the parsing side, it is also supposed to be a SyntaxError if a const declaration lacks an initializer, but I think that's an easier fix.