Support top-level await

[JasonBarnabe]

https://violentmonkey.github.io/api/metadata-block/#top-level-await
rubyjs/mini_racer#353

[cyfung1031]

For TM, this is supported by default. No need to have @top-level-await

Tampermonkey/tampermonkey#1820

So Greasy Fork shall just ignore this meta, because this is no meaning to some userscript managers.