Utilise package extensions to add features

[TorkelE]

Julia 1.9 added package extensions, allowing us to have features that are dependent of other packages, without affecting people not interested in these: https://github.com/JuliaLang/julia/blob/release-1.9/NEWS.md

It would allow us to;

1. Add wrapper functions around HomotopyContinuation, automating some of the stuff required to find steady states. For systems without conservation laws, we could simply have a function hc_ss(rs, ps) that gives the steady states.
2. Makie-related plotting features. primarily useful for spatial systems.
   Previously I had assumed that these would need to be put elsewhere, but now we could actually consider integrating it into Catalyst.

[TorkelE]

While not all extension are merged yet, this has now become established use.