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/Interpreter.cpp

@@ -27,8 +27,16 @@ void Interpreter::run(Bytecode::Block const& block)
 
     m_registers.resize(block.register_count());
 
-    for (auto& instruction : block.instructions())
+    size_t pc = 0;
+    while (pc < block.instructions().size()) {
+        auto& instruction = block.instructions()[pc];
         instruction.execute(*this);
+        if (m_pending_jump.has_value()) {
+            pc = m_pending_jump.release_value();
+            continue;
+        }
+        ++pc;
+    }
 
     dbgln("Bytecode::Interpreter did run block {:p}", &block);
     for (size_t i = 0; i < m_registers.size(); ++i) {