feat: allow environment to host a BottomRightPanel button

[pankgeorg]

The new progress bar introduces a super nice place to add non notebook information, for example about the environment the notebook is running on.

This PR introduces a way to hook into this space.

[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/fonsp/Pluto.jl", rev="pg/env-extend-side-panel")
julia> using Pluto

[fonsp]

Sorry I missed this PR! Do we still want this?

[pankgeorg]

Sorry I missed this PR! Do we still want this?

We will probably want this soon, yes! But we're not ready atm