Support const within classes

It would be nice to have class-scoped consts which don't pollute the global scope.

const THIS_IS_GLOBALLY_AVAILABLE = "abc"

class Abc
    const A_LOCALLY_SCOPED_CONST = "def"

    function getMeAValue()
        return A_LOCALLY_SCOPED_CONST
    end function
end class

print THIS_IS_GLOBALLY_AVAILABLE         ' > "abc"
print new Abc().getMeAValue()            ' > "def"
print new Abc().A_LOCALLY_SCOPED_CONST   ' > invalid
print Abc.A_LOCALLY_SCOPED_CONST         ' > invalid (I think)

Use the examples in the issue as the behavioral specification. Start by locating the compiler's existing handling for global const declarations and class members, then trace how scope and member visibility are validated. Done means class-scoped consts work inside their class without entering global scope, while the shown external accesses remain invalid.