fix(react-is): support legacy React elements (react.element)

[Liuwei1125]

Bug Fix Summary

Issue: #36409 - react-is@19 does not recognise elements from React 18

Problem

React 19 changed $$typeof from Symbol('react.element') to Symbol('react.transitional.element'). The react-is@19 package only checks for the new symbol, breaking backward compatibility with React 17/18 elements.

Solution

Update isElement() and typeOf() in packages/react-is/src/ReactIs.js to also recognize REACT_LEGACY_ELEMENT_TYPE.

Changes

1. ReactIs.js: Import REACT_LEGACY_ELEMENT_TYPE and add it to:

   • typeOf() switch statement
   • isElement() check

2. ReactIs-test.js: Add test case should identify legacy elements from older React versions

Testing

// Before fix
reactIs.isElement(react18Element) // false

// After fix
reactIs.isElement(react18Element) // true

---

This fix ensures tools that upgrade to react-is@19 (like jest's pretty-format) can still identify elements created by React 17/18 without requiring users to upgrade React.

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[Liuwei1125]

I have signed the Meta CLA. Please re-check.

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