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/Bytecode/Generator.h

@@ -7,6 +7,7 @@
 #pragma once
 
 #include <AK/OwnPtr.h>
+#include <LibJS/Bytecode/Label.h>
 #include <LibJS/Forward.h>
 
 namespace JS::Bytecode {
@@ -26,6 +27,8 @@ public:
         return *ptr;
     }
 
+    Label make_label() const;
+
 private:
     Generator();
     ~Generator();