◐ Spin Pkg terminal spinners with CSS for performance

[fonsp]

#2500 introduced two performance issues:

1. High network pressure and latency Fancyprint pkg logs: not throttled on slow connection #2739
2. The terminal outputs are stored in the state, and are included in every codemirror as a compartment value (because the terminal text is displayed in the Pkg popup). This was causing lots of expensive re-renders. According the the profile, most time was spent in the CM6 compartment/extension state management.

The solution in this PR is to replace all 4 progress characters with ◐ on the server side (before diffing). This fixes both issues because the updates are now much less frequent.

To still get a spinning animation, we search for the ◐ character, give it a class and make it spin with a CSS animation. The result is almost identical!

Before

Schermopname.2024-01-22.om.11.06.57.mov

After

Schermopname.2024-01-22.om.11.27.16.mov

You can see that there are much fewer messages from the server:

Schermopname.2024-01-22.om.11.33.11.mov

fix #2739

[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="pkg-terminal-spinners-with-css")
julia> using Pluto

[fonsp]

I'm really proud of this PR 😙