LibJS: Add basic support for while loops in the bytecode engine

This introduces two new instructions: Jump and JumpIfFalse.
Jumps are made to a Bytecode::Label, which is a simple object that
represents a location in the bytecode stream.

Note that you may not always know the target of a jump when adding the
jump instruction itself, but we can just update the instruction later
on during codegen once we know where the jump target is.

The Bytecode::Interpreter now implements jumping via a jump slot that
gets checked after each instruction to see if a jump is pending.
If not, we just increment the PC as usual.

Userland/Libraries/LibJS/AST.h

@@ -370,6 +370,7 @@ public:
 
     virtual Value execute(Interpreter&, GlobalObject&) const override;
     virtual void dump(int indent) const override;
+    virtual Optional<Bytecode::Register> generate_bytecode(Bytecode::Generator&) const override;
 
 private:
     NonnullRefPtr<Expression> m_test;