More embedded Pkg API

[fonsp]

Includes:

• show_help::Bool kwarg for activate_notebook_environment to disable the help banner
• New function has_notebook_environment(path::String) or has_notebook_environment(notebook::Notebook) to check if a notebook as embedded pkg files

[github-actions]

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="more-notebook-pkg-api")
julia> using Pluto