LibJS: Implement "else" parsing

We can now handle scripts with if/else in LibJS. Most of the changes
are about fixing IfStatement to store the consequent and alternate node
as Statements.

Interpreter now also runs Statements, rather than running ScopeNodes.

Libraries/LibJS/Forward.h

@@ -40,6 +40,7 @@ class Interpreter;
 class Object;
 class PrimitiveString;
 class ScopeNode;
+class Statement;
 class Value;
 enum class DeclarationType;