Disallow top-level `await`

[pomerantsev]

I noticed that older versions of Webpack don’t support it, so build in styled-components fails.
We don't have top-level await yet, but this may be a time bomb.

[pomerantsev]

Update: Biome doesn't have a "no top-level await" rule, and at this time Biome doesn't have a plugin system, which means that I cannot write a custom rule or use one written by someone else: biomejs/biome#1762

The only viable option seems to be to introduce ESLint, which would be scanning the files in addition to Biome. I'm not willing to do that just yet, but I may change my mind later.