LibJS: Implement null and undefined literals

Author: https://github.com/0xtechnobabble
Commit: cfd710eb31
Pull-request: https://github.com/SerenityOS/serenity/pull/1470
Reviewed-by: https://github.com/awesomekling
Reviewed-by: https://github.com/oriko1010

Libraries/LibJS/AST.h

@@ -380,6 +380,34 @@ private:
     String m_value;
 };
 
+class NullLiteral final : public Literal {
+public:
+    explicit NullLiteral()
+    {
+    }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "NullLiteral"; }
+
+    String m_value;
+};
+
+class UndefinedLiteral final : public Literal {
+public:
+    explicit UndefinedLiteral()
+    {
+    }
+
+    virtual Value execute(Interpreter&) const override;
+    virtual void dump(int indent) const override;
+
+private:
+    virtual const char* class_name() const override { return "UndefinedLiteral"; }
+};
+
 class Identifier final : public Expression {
 public:
     explicit Identifier(String string)