[core] fix(ResizeSensor): compatibility with React 18

[PurkkaKoodari]

Fixes #5313 & another bug

Checklist

• Includes tests: This is not testable easily without testing against React 18, which would be a pretty big change given that the build is currently hard-coded against React 16.
• Update documentation: I didn't touch the docs, as this doesn't change the user-facing API at all.

(If you think this needs tests or docs, I'll update the PR.)

Changes proposed in this pull request:

Firstly, this fixes #5313 by defining an explicit return type for render().

The difference between React 18 and 16 here seems to be that React 18 specifies ReactFragment = Iterable<ReactNode>, while React 16 specifies {} | ReactNodeArray. The definition of only drops the array, so the compiled type declaration is an union of what's left of ReactNode. That breaks when the newer @types/react doesn't allow {}. Specifying ReactNode should allow the union at compile time, and then the compiled render(): ReactNode is compatible with any version of React.

A quick grep for React.Children.only suggests that packages/table/src/common/loadableContent.tsx and packages/table/src/interactions/draggable.tsx are also affected, but I didn't have time to test them right now.

Secondly, this makes ResizeSensor work with React 18 StrictMode and its "reusable state", which essentially runs effects twice - against their own docs for componentWillUnmount. (The docs make this very enjoyable to debug.)

The current code assumes that the component is dumped after componentWillUnmount so it doesn't bother to clear this.element. This permanently disables the observer since observeElement will then think the element is already being observed. I added this.element = null to make it "reusable".

Reviewers should focus on:

If this breaks something subtle I didn't notice, I guess?

[palantirtech]

Thanks for your interest in palantir/blueprint, @PurkkaKoodari! Before we can accept your pull request, you need to sign our contributor license agreement - just visit https://cla.palantir.com/ and follow the instructions. Once you sign, I'll automatically update this pull request.

[adidahiya]

Sweet, this looks like a clean solution to the problem, thanks @PurkkaKoodari!

[PetrusAsikainen]

@adidahiya Seems this change never made it to v5?

[adidahiya]

@PetrusAsikainen good catch! Fixing that here: #6191