REPL mode

[tlienart]

Requires primarily to parse a code block into split stuff. Can look at https://github.com/JuliaDocs/Documenter.jl/blob/6d633fbfb01407f44434f9cfa84e67586b6e8957/src/Utilities/Utilities.jl#L96-L150

but in short would amount to doing a gradual parse of the code block (which is already done I think), then instead of executing the block in one shot, execute each expression one by one.