LibJS: Add Uint8ClampedArray :^)

This is a special kind of byte array that clamps its values to 0...255
It will be used for HTML ImageData objects.

I made Object::put_by_index() and get_by_index() virtual for this.
We'll probably need to make non-numeric property name lookups virtual
as well, but this solves my current problem well enough.

Libraries/LibJS/Forward.h

@@ -69,6 +69,7 @@ class PrimitiveString;
 class ScopeNode;
 class Shape;
 class Statement;
+class Uint8ClampedArray;
 class Value;
 enum class DeclarationKind;