Frontmatter GUI: multiple authors

[fonsp]

We already supported multiple authors in frontmatter metadata and in the featured notebook viewer, and now there is also a GUI for inputting this information!

You can even add custom fields to authors!

How it's stored in the file

### A Pluto.jl notebook ###
# v0.19.27

#> [frontmatter]
#> image = "https://github.com/JuliaPluto/featured/assets/6933510/f8bec040-bb45-45c5-bedc-b2528e175c4c"
#> title = "Fun with 🌒Emojis and Julia"
#> date = "2023-07-25"
#> tags = ["PlutoUI", "animation", "simple", "clock", "Unicode"]
#> description = "Making use of 🌒 emojis and PlutoUI.clock for a simple animation"
#> license = "Unlicense"
#> 
#>     [[frontmatter.author]]
#>     name = "Boshra"
#>     url = "https://github.com/ariguiba"
#>     [[frontmatter.author]]
#>     name = "Benedikt"
#>     favourite_animal_tags = ["bat", "hummingbird?"]
#>     url = "https://github.com/behinger"

using Markdown
using InteractiveUtils

[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="multiple-authors-frontmatter-GUI")
julia> using Pluto