LibJS: Add basic support for "continue" in the bytecode VM

Unlike the convoluted unwind-until-scope-type mechanism in the AST
interpreter, "continue" maps to a simple Bytecode::Op::Jump here. :^)

We know where to jump based on a stack of "continuable scopes" that
we now maintain on the Bytecode::Generator as we go.

Note that this only supports bare "continue", not continue-with-label.

Userland/Libraries/LibJS/AST.h

@@ -1300,6 +1300,7 @@ public:
     }
 
     virtual Value execute(Interpreter&, GlobalObject&) const override;
+    virtual Optional<Bytecode::Register> generate_bytecode(Bytecode::Generator&) const override;
 
     const FlyString& target_label() const { return m_target_label; }