Combine createElement and JSX modules (#28320)

Depends on:

- #28317 

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There's a ton of overlap between the createElement implementation and
the JSX implementation, so I combined them into a single module.

In the actual build output, the shared code between JSX and
createElement will get duplicated anyway, because react/jsx-runtime and
react (where createElement lives) are separate, flat build artifacts.

So this is more about code organization — with a few key exceptions, the
implementations of createElement and jsx are highly coupled.