Write more custom ESLint plugins

[toolness]

It might be nice to write some custom ESLint plugins that ensure things like:

• not importing anything in a tests directory from production code, Update: done in #1444!
• not importing anything in norent from the regular justfix site,
• accidentally importing <Trans> from @lingui/react instead of @lingui/macro Update: done in #1553!
• we alwaysawait functions that return promises (this kind of plugin may already exist), or at least do it for testing functions that start with wait.

Aside from showing up as errors in CI, I think they would show up inline in VSCode, which would be nice.