Run spx in standalone thread

[nighca]

To avoid blocking page interaction.

By default, iframe script runs in the same thread as the parent page. See details in https://stackoverflow.com/questions/11510483/will-a-browser-give-an-iframe-a-separate-thread-for-javascript.

Running spx / godot in a separate thread (maybe by utilizing web worker) can improve page interactivity.

Related: https://github.com/goplus/builder/issues/1496