Add TagBot and CompatHelper actions

[ablaom]

These are very helpful additions for Julia pkg maintenance.

https://github.com/JuliaRegistries/TagBot : tag new releases automatically

https://github.com/JuliaRegistries/CompatHelper.jl: automatically generate PR's when dependencies break existing compat bounds

You may also take a look at PkgTemplates.jl for other ideas; see in particular this.

[boborbt]

Latest commit adds TAgBot and CompatHelper. I had a look at the references you gave me, but CI seems to add little to what already is in place (if I'm not mistaken adds actions for tests and doc generation, which I already have).

If you do not have any other specific requirements I would considered closed this issue.