:sparkle: Isolated Cell Layout builder :sparkle:

[ghaetinger]

This PR adds a small checkbox button on the side of every cell that contains an output. When selected, the cell will be flagged as isolated. All of the cells marked as such will be layed out in an Isolated Cell Layout when a button in the export menu is clicked.

Selections are saved much like code folding, but for means of practicality, I added an identifier to the right side of the UUID, which I fell like is a big no-no.

[github-actions[bot]]

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/ghaetinger/Pluto.jl", rev="main")
julia> using Pluto