Add shared utility isValidElementType to React

[ArsiyaTasleem90250]

What does this PR do?

This PR adds a new shared utility function isValidElementType under packages/shared/.
The function helps check whether a given value can be used as a valid React element type (string, function component, or component object).
Includes unit tests validating behavior for strings, functions, null/undefined, and objects.

Why is this useful?

In various parts of React codebase or third-party libraries, we often need to check if a variable is a valid element type before rendering or passing it further.
This helper standardizes that logic in one place, avoiding duplication and improving code readability and safety.

Checklist (as per React contributing guidelines)

• Added new file with correct license header
• Added corresponding test file
• All tests pass (yarn test)
• Code is formatted and linted (yarn prettier, yarn lint)
• Flow type checks pass (yarn flow)
• No breaking changes to existing API

Thank you for considering this contribution.

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