feat: empty form editor state

[Skaiir]

Closes #336

[pinussilvestrus]

We need to update some screenshots for the visual regression tests as we have empty forms over there - that is expected. Let me take this 📸

https://github.com/bpmn-io/form-js/actions/runs/5590547092/job/15141869294?pr=725

[pinussilvestrus]

For me, it looks clean 🥇

I'm adding @RomanKostka and @christian-konrad to have a look as well, please find here the demo: https://demo-336-proper-empty-state--camunda-form-playground.netlify.app/

[christian-konrad]

Finally! Let's go

[RomanKostka]

awesome!
one thing i would like to change:
add a . after each sentence - this makes the message clearer to read.
Example:

One aditional thing that is not related to this issue:
We should probably try to align the BPMN.IO logo in the definition + preview window so that is stays at the same height.
Examples: