Add reveal.js support

[virgile-dev]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
Some users of the pad used the reveal.js support to make presentations.
This a geeky feature but it really aligns with our values of putting content first.
With reveal.js you make presentation using only text and don't do any formatting.

Describe the solution you'd like
Create BlockNote plugin that makes it compatible with reveal.js