fix sdist

[rmorshea]

_{By submitting this pull request you agree that all contributions to this project are made under the MIT license.}

Issues

Closes: https://github.com/reactive-python/reactpy/discussions/1180 Closes: https://github.com/reactive-python/reactpy/pull/1183

Solution

Create a link to the JS directory inside src/py/reactpy and include the JS dir as an sdist-only artifact

Checklist

• [x] Tests have been included for all bug fixes or added functionality.
• [ ] The changelog.rst has been updated with any significant changes.

[rmorshea]

~@Archmonger I'm having some trouble with this on Windows, do you think you can try this branch out and see what's happening. In the meantime I'm gonna purchase a Windows license and install it on an old laptop so I can test this stuff myself going forward.~

Never mind. Got past the windows issue.