Markdown snips

Allow to render markdown in our labbook!

We will most likely only some or omit extended features. Maybe something like https://spec.commonmark.org/0.31.2/ is a good target but omitting images and raw

TODOs: Rending markdown to canvas is mostly done but the frontend integration is mostly missing. I.e. sending markdown via the api works but there is no user interface.

• Render ast as dom for mouse hover
• Add an editor for markdown text to the pageseditor