Pulled out from https://github.com/microsoft/playwright/pull/31900
I stumbled over `React.Children`, because it's the first time I saw that
used. https://react.dev/reference/react/Children lists `React.Children`
it as "Legacy" and mentions it's uncommon. Also, the fact that SplitView
only displays its first two children, and all others are silently
discarded, can be a surprise to some.
By separating things out into `sidebar` and `main`, not only do we give
the two elements names (otherwise one needs to remember that sidebar is
always the first child), but we also prevent any "third children" from
being dropped.
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.
