⌨️ Keyboard shortcut for folding cell

[fonsp]

Fix #2917

The implemented keyboard shortcut is Ctrl/Cmd + K

MacOS: ⌥⌘[ to fold, ⌥⌘] to unfold
Windows/Linx: Ctrl+Shift+[ to fold, Ctrl+Shift+] to unfold

Loosely matching VS Code

You can do this while editing a cell, or with multiple cells selected

I also made #2920 to make it more clear that folding the cell does not hide the code yet because you are still focused on the codemirror.

[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="keymap-fold")
julia> using Pluto

[fonsp]

Nooo Cmd+K is not so good, i misunderstood the vs code thing

But the shortcuts for "folding" "unfolding" code sections might actually be a good fit. I don't think we ever want to implement code folding (ie within cells) in Pluto because cells should be fairly small, so using it for folding cells might be a good use for the shortcuts.

Ctrl+Shift+[ ] on windows linux