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.cpp

@@ -42,4 +42,9 @@ Register Generator::allocate_register()
     return Register { m_next_register++ };
 }
 
+Label Generator::make_label() const
+{
+    return Label { m_block->instructions().size() };
+}
+
 }