LibJS: Implement basic support for the "delete" operator

It turns out "delete" is actually a unary op :)
This patch implements deletion of object properties, it doesn't yet
work for casually deleting properties from the global object.

When deleting a property from an object, we switch that object to
having a unique shape, no longer sharing shapes with others.
Once an object has a unique shape, it no longer needs to care about
shape transitions.

Libraries/LibJS/AST.h

@@ -400,6 +400,7 @@ enum class UnaryOp {
     Minus,
     Typeof,
     Void,
+    Delete,
 };
 
 class UnaryExpression : public Expression {