LibJS: Some more opcodes for the bytecode VM

- NewString (allocates a new PrimitiveString from the GC heap)
- GetVariable (retrieves a variable in the current scope)
- SetVariable (assigns a variable in the current scope)

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -6,6 +6,7 @@
 
 #pragma once
 
+#include <AK/FlyString.h>
 #include <LibJS/Bytecode/Instruction.h>
 #include <LibJS/Bytecode/Register.h>
 #include <LibJS/Heap/Cell.h>
@@ -49,4 +50,55 @@ private:
     Register m_src2;
 };
 
+class NewString final : public Instruction {
+public:
+    NewString(Register dst, String string)
+        : m_dst(dst)
+        , m_string(move(string))
+    {
+    }
+
+    virtual ~NewString() override { }
+    virtual void execute(Bytecode::Interpreter&) const override;
+    virtual String to_string() const override;
+
+private:
+    Register m_dst;
+    String m_string;
+};
+
+class SetVariable final : public Instruction {
+public:
+    SetVariable(FlyString identifier, Register src)
+        : m_identifier(move(identifier))
+        , m_src(src)
+    {
+    }
+
+    virtual ~SetVariable() override { }
+    virtual void execute(Bytecode::Interpreter&) const override;
+    virtual String to_string() const override;
+
+private:
+    FlyString m_identifier;
+    Register m_src;
+};
+
+class GetVariable final : public Instruction {
+public:
+    GetVariable(Register dst, FlyString identifier)
+        : m_dst(dst)
+        , m_identifier(move(identifier))
+    {
+    }
+
+    virtual ~GetVariable() override { }
+    virtual void execute(Bytecode::Interpreter&) const override;
+    virtual String to_string() const override;
+
+private:
+    Register m_dst;
+    FlyString m_identifier;
+};
+
 }