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/Bytecode/Generator.cpp

@@ -47,4 +47,19 @@ Label Generator::make_label() const
     return Label { m_block->instructions().size() };
 }
 
+Label Generator::nearest_continuable_scope() const
+{
+    return m_continuable_scopes.last();
+}
+
+void Generator::begin_continuable_scope()
+{
+    m_continuable_scopes.append(make_label());
+}
+
+void Generator::end_continuable_scope()
+{
+    m_continuable_scopes.take_last();
+}
+
 }