Julia 1.3 support

[mtfishman]

It looks like this line: https://github.com/JuliaPlots/GraphMakie.jl/blob/649f1aec493bc57fdd3ca66e57d6b5eaaf946af4/src/beziercurves.jl#L55
is using syntax introduced in Julia 1.4:
https://github.com/JuliaLang/julia/blob/master/HISTORY.md#new-language-features-3

It seems like in general Makie supports Julia 1.3:
https://github.com/JuliaPlots/Makie.jl/blob/2faa3d75733689f2d3e802b1f623d0f90343005a/Project.toml#L91

We are using GraphMakie as a backend in our new tensor network visualization backend in ITensors.jl. Generally we try to support Julia 1.3 so I think it would be nice if GraphMakie did as well.

[hexaeder]

A quick search showed that I'm using array[begin] and array[end] in a few places. Feel free to open a PR to change this to array[firstindex(array]) and so forth. Then we should also add those old Julia versions to the tests (the tests are not great at all but at least they will run plot command a few times)...

[mtfishman]

Sure, I can make a PR.

Around when I raised this issue Julia 1.6 was switched to LTS (and some big projects like SciML have switch to only supporting Julia 1.6), so this may not be so important, but for the time being I can still fix the issue to make it work back to Julia 1.3 since Makie support 1.3.

[hexaeder]

Yeah i was thinking the same. If you need it for ITensors go for it I'm happy to merge. But maybe it is a good time to get rid of the support of those ancient versions. I think with SciML raising the bounds to 1.6 there won't be a lot of 1.3 users left....