LibJS: Add placeholder bytecode block sealing mechanism

After compiling bytecode, we should mark the memory read-only.
This currently does not work because it breaks instruction destruction.

I'm adding this anyway with a FIXME so we don't forget about it. :^)

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -28,6 +28,7 @@ OwnPtr<Block> Generator::generate(ASTNode const& node)
     Generator generator;
     [[maybe_unused]] auto dummy = node.generate_bytecode(generator);
     generator.m_block->set_register_count({}, generator.m_next_register);
+    generator.m_block->seal();
     return move(generator.m_block);
 }