Support [sources] section in Project.toml for Julia 1.11

[j-fu]

Hi, in Julia 1.11, Project.toml can have a [sources] section which allows to specify a path or an url for a package from the [deps] section, this was announced at JuliaCon, see also
https://github.com/JuliaLang/Pkg.jl/blob/master/CHANGELOG.md#pkg-v111-release-notes

Could this be somehow supported in Pluto ? Something like

package_source(pkgname, pkgsource)

An alternative might be some tools which help to manipulate notebook environments. May be they even are already there but may be not marked as public interface.

[fonsp]

Thanks for the suggestion! [sources] is a really good fit for Pluto!

This gave me the idea for #3099 :)