Change paragraph representation in the editor to div[role="paragraph"] (LEAN-738)

In this PR will revert work on the idea of mixing inline node with block by creating fragment node as it's hard to change the structure of paragraph node as this will have effect on the editor commands and creating nodes by prosemirror assume that paragraph will include just inline content.

and will fix issue of block node with paragraph by moving them out as @teemukoivisto suggest, which make sense and easier.