Fix schema error

[steff456]

Fixes #57

This PR,

• [x] Removes unnecessary lerna option
• [x] Removes schema from main package.json
• [x] Removes schema folder from main package
• [x] Avoid setting registry inside the plugin of each theme

[github-actions[bot]]

:point_left: Launch a binder notebook on branch Quansight-Labs/jupyterlab-accessible-themes/fix-schema _{Comment updated on 2023-07-13T18:51:15.627Z}

[steff456]

I was able to remove those lines. I think this PR is ready for merge!

[trallard]

Linting, links, and formatting issues resolved, so ready to merge 🚀