embed_display: await inner scripts to finish

[fonsp]

Fix https://github.com/JuliaPluto/PlutoUI.jl/issues/236

After this PR, the <script> element that powers the embed_display will await all embedded <script> elements to finish.

TODO:

• [x] frontend test would be great but takes some effort
• [ ] I had to use a requestIdleCallback to keep this from getting crazy complicated, which might lead to slower rendering when using lots of embed_display in one cell... needs to be investigated.

[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="embed_display-await-inner-scripts")
julia> using Pluto

[fonsp]

Hmpf I tricked myself into thinking that this PR worked because of #1776 , it's not actually working yet.