LibJS: Implement bytecode generation for UpdateExpression :^)

Added Increment and Decrement bytecode ops to support this. Postfix
updates use a temporary register to preserve the original value.

Note that this patch only implements Identifier updates. Member
expression updates are a TODO.

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -373,6 +373,28 @@ public:
     String to_string() const;
 };
 
+class Increment final : public Instruction {
+public:
+    Increment()
+        : Instruction(Type::Increment)
+    {
+    }
+
+    void execute(Bytecode::Interpreter&) const;
+    String to_string() const;
+};
+
+class Decrement final : public Instruction {
+public:
+    Decrement()
+        : Instruction(Type::Decrement)
+    {
+    }
+
+    void execute(Bytecode::Interpreter&) const;
+    String to_string() const;
+};
+
 }
 
 namespace JS::Bytecode {