Remove Requires and implement a Makie extension instead

[mohamed82008]

Summary

• This PR implements Makie support as an extension instead of relying on Requires.jl.
• I also define the visualize function now at the top level in TopOpt.jl and overload it in the extension. This means that a single visualize function is overloaded for both continuum and truss problems. This function is also exported by default.
• This PR also avoids loading GLMakie in the extension and instead only loads and depends on Makie. Users can choose which backend they want to use themselves by loading GLMakie, WGLMakie or CairoMakie at the top level of their code.
• Plotting code is now tested using CairoMakie as a backend on CI which is easy to setup. This is just to avoid bit rotting of code.

[yijiangh]

From @mohamed82008 's description, the intention of this PR looks good to me. But I don't understand how TopOptMakieExt gets loaded. For example, in the beso script, I don't see you import TopOptMakieExt, how does Julia dispatch visualize to the right function?

[mohamed82008]

Extension packages in Julia get loaded automatically just like Requires worked in the past. So when I call using Makie and then using TopOpt (order doesn't matter), Julia will load the TopOptMakieExt module automatically. This module extends the otherwise empty visualize function with no methods by default. So if you try to use visualize without loading Makie first, you will get an error. A nicer experience is perhaps to define a fallback that throws an informative error asking the user to load Makie:

function visualize(arg::T; kwargs...) where {T}
    throw(ArgumentError("`visualize` is not defined for input type $T. This may be because the input to the function is incorrect or because you forgot to load `Makie` in your code. You can load `Makie` with `using Makie`."))
end

[mohamed82008]

Note that it is just a single function TopOpt.visualize. The extension module defines 2 extra methods for this function.

[mohamed82008]

The latest commit implements a more informative error message.

[yijiangh]

Extension packages in Julia get loaded automatically just like Requires worked in the past. So when I call using Makie and then using TopOpt (order doesn't matter), Julia will load the TopOptMakieExt module automatically. This module extends the otherwise empty visualize function with no methods by default. So if you try to use visualize without loading Makie first, you will get an error. A nicer experience is perhaps to define a fallback that throws an informative error asking the user to load Makie:

function visualize(arg::T; kwargs...) where {T}
    throw(ArgumentError("`visualize` is not defined for input type $T. This may be because the input to the function is incorrect or because you forgot to load `Makie` in your code. You can load `Makie` with `using Makie`."))
end

Good to know! Do we have to declare extension packages somewhere, or we simply need to put it in to the ext folder?

[mohamed82008]

@yijiangh these 2 new Project.toml sections are where we declare the extension packages