fix: Inline isFragment function to remove react-is dependency

[amanabiy]

Description

This PR inlines the isFragment function in the internal vendored react-keyed-flatten-children implementation to completely eliminate the react-is dependency. This builds on PR #3997.

Why?

React 19 renamed the element symbol from react.element to react.transitional.element to detect version mismatches in React and react-is (React PR #28813). This means we can't use react-is if we want to support both React 19 and maintain backward compatibility. Therefore, we need to replace the isFragment function with an internal implementation to make it work with both forward and backward compatibility.

Here is a minimal example of the problem using sandbox

Changes

It makes it easy for review if you go through the commits

• The first commit reverses this PR #3997
• The second commit makes this isFragment inline removing react-is
• The third commit adds bigint support to handle bigint as a valid child type for React 19 compatibility (React PR #24580). TypeScript will error on bigint children with React <19 before reaching this conditional statement.

How I tested it:

• Verified across multiple React versions from 16 to 19 using a simple application to test the flatten works as expected.

Related links, issue #, if available: n/a

How has this been tested?

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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.18%. Comparing base (e494891) to head (50f72ca).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4220   +/-   ##
=======================================
  Coverage   97.18%   97.18%           
=======================================
  Files         883      883           
  Lines       25870    25870           
  Branches     9347     9347           
=======================================
  Hits        25141    25141           
  Misses        682      682           
  Partials       47       47           

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