Addresses https://github.com/microsoft/playwright/issues/31863. This PR
is chonky, but the individual commits should be easy to review. If
they're not, i'm happy to break them out into individual PRs.
There's two main things this does:
1. Remove some unused imports
2. Add a `clsx`-inspired helper function for classname templating
I wasn't able to replace `ReactDOM.render` with `ReactDOM.createRoot`.
This is the new recommended way starting with React 18, and the existing
one is going to be deprecated at some point. But it somehow breaks our
tests, i'll have to investigate that separately.
