chore: Vendor react-keyed-flatten-children dependency

[TrevorBurnham]

Description

We're unable to upgrade to the latest versions of react-keyed-flatten-children because of how it's packaged: We have Jest customers that are allergic to ESM. So we're stuck on an old version.

From #3080:

It is such a small utility that sometimes I wonder if it's worth inlining into the package…

This PR does exactly that: It adds the dep to our vendor/ dir and adds an appropriate peer dependency on react-is. That peer dependency should be a significant efficiency improvement for many of our customers: The version of react-keyed-flatten-children we were using depended on react-is@^18.2.0, which meant that customers with any higher or lower version of react-is would wind up with two copies of that dependency in their bundle.

How has this been tested?

All unit tests pass.

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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