API for widgets to return possible bind values

[Pangoraw]

See:

• Precompute all possible slider states JuliaPluto/PlutoSliderServer.jl#29
• slider and checkbox export possible values JuliaPluto/PlutoUI.jl#147

(We will merge this after the functionality in PlutoSliderServer is finished)

[fonsp]

Can use the new stuff from #1612 !

[fonsp]

I just realised: the possible bind values should stay in the notebook process, because they might not be serializable...... You only want the outside process to know the length, and to say e.g. "set bound variable x to the 17th possible value". Will come back to this next week!

[Pangoraw]

I think the collected values should correspond to the JavaScript possible values so they should be serializable. If we collect Julia values then the Message pack hashes for the files will be wrong. The Julia can still be found by using the js->jl transform. Also see JuliaPluto/PlutoUI.jl#152.

[fonsp]

Totally right! I should change this https://docs.juliahub.com/AbstractPlutoDingetjes/UHbnu/1.1.0/#:~:text=If%20you%20are%20also%20using%20transform_value%20for%20your%20widget%2C%20then%20the%20values%20returned%20by%20possible_values%20should%20be%20the%20values%20after%20transformation.

EDIT: changed in JuliaPluto/AbstractPlutoDingetjes.jl@a075180