Move Plots.jl functionality into an extension

[sloede]

With #2214 we move to Julia v1.10 as the minimum supported version and can thus rely on package extensions instead of using Requires.jl. While all other optional features are already available as a package extension, our Plots.jl functionality is currently only available via Requires.jl. It would be good to this to an package extension as well, and then dump Requires.jl as a dependency.