Update examples to use lowercase reactive.{Calc,Effect,Value}

[schloerke]

From #822

This PR changes those names to lowercase, and adds aliases so that the old capitalized names still work.

This PR does not change any examples because if users on a previous version of shiny were to use the example code, it wouldn't work for them, and it would be unclear to them why. In a future release, we should update the examples to use the new lowercase names.

[wch]

Note that we also need to update the examples in the shinylive repo and py-shiny-site.