feat: supports top-level await and namespace isolation

[ACTCD]

resolve #749 and supports top-level await in user script

[ACTCD]

Supporting top-level await may cause further delays in user script execution especially in document-start requirements.

But since document-start itself cannot be implemented in current Safari, this issue is not a fatal regression.

Really solving all issues will require waiting for the implementation of the UserScripts API (#756), which provides a true document-start, and that we will no longer provide an async wrapper.