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.

[ShahuK283]

Hi @sloede,

This issue has been open for a little while, I just wanted to double-check: is this still something that needs to be done? If so, I am happy to start moving the Plots.jl functionality over to an ext/ directory, updating the Project.toml, and officially dropping Requires.jl.

Let me know if I am clear to start!