LibJS: Allow the choice of a scope of declaration for a variable (#1408)

Previously, we were assuming all declared variables were bound to a
block scope, now, with the addition of declaration types, we can bind
a variable to a block scope using `let`, or a function scope (the scope
of the inner-most enclosing function of a `var` declaration) using
`var`.

Libraries/LibJS/Forward.h

@@ -37,5 +37,6 @@ class Object;
 class PrimitiveString;
 class ScopeNode;
 class Value;
+enum class DeclarationType;
 
 }