Replace `JSX` global with explicit `ReactElement`

[remcohaszing]

Initial checklist

• [x] I read the support docs
• [x] I read the contributing guide
• [x] I agree to follow the code of conduct
• [x] I searched issues and couldn’t find anything (or linked relevant results below)
• [x] If applicable, I’ve added docs and tests

Description of changes

This makes the react-markdown types compatible with React 19. Dependencies still rely on the JSX namespace, but let’s trackle one thing at a time.

The better return type would be ReactNode, not ReactElement. But that breaks compatibility with TypeScript < 5.1. Let’s do that in a semver major.

[github-actions[bot]]

Hi! This was closed. Team: If this was merged, please describe when this is likely to be released. Otherwise, please add one of the no/* labels.

[remcohaszing]

The second commit was needed, because apparently type-coverage started failing.

[wooorm]

It was likely possible to side-step that with less changes: It didn’t get ReturnType<typeof Markdown>, for some reason, presumably JSX.Element, ReactNode, ReactElement, would’ve worked. Or a comment to ignore that one line?