Line wrapping: support spaces indenting

[fonsp]

This now works identically with spaces and tabs:

Try this Pull Request!

Open Julia and type:

julia> import Pkg
julia> Pkg.activate(temp=true)
julia> Pkg.add(url="https://github.com/JuliaPluto/Pluto.jl", rev="awesome-line-wrapping-spaces")
julia> using Pluto