[deps] Update to React v18

[bgptr]

The first step toward #3842

This diff updates to React v18 and enjoys the benefit of automatic performance improvement. It does not take further steps toward the new features, but it will be a good starting point.

The biggest issue was the react-hot-loader, which is not maintained anymore and is not compatible with v18. I successfully replaced it with React Fast Refresh though, which is the recommended way.

@testing-library/* libraries also have been updated and the tests are fixed.

On top of this, we can gradually adopt the new features:

• use the new APIs for concurrent rendering:
  • startTransition
  • useDeferredValue
• update to strict mode

Tested on Linux, and macOS (amd64).

[dajohi]

@bgptr can you please rebase this?

[bgptr]

@dajohi Sure. I will do it asap.