Streamsync

Minor improvements on the first fully-featured version of Streamsync.

- `pandas` and `plotly.express` are now optional dependencies of the package, installable with `streamsync[ds]`
- `build` and `test` optional dependencies for those developing Streamsync
- Command line usability improvements
- Updated versions of UI dependencies

Streamsync is no longer a POC!

It's now a **fully-featured data apps framework**. Build user interfaces using a visual editor; write the backend code in Python. Check out a [live demo](https://hello.streamsync.cloud/) of a running app. Visit the [documentation](https://streamsync.cloud) website to learn more.

Thank you everyone for your support.