Add GitHub actions

[odow]

You should add some continuous integration jobs.

Ipopt has some examples to follow:

• TagBot: https://github.com/jump-dev/Ipopt.jl/blob/master/.github/workflows/TagBot.yml. This will make a GitHub release when you tag a new version. Releases look like https://github.com/jump-dev/Ipopt.jl/releases/tag/v1.6.2.
• Tests: https://github.com/jump-dev/Ipopt.jl/blob/master/.github/workflows/ci.yml. Nice to see you have a test/runtests.jl file, but this job will check that the tests pass each commit.
• Formatting: https://github.com/jump-dev/Ipopt.jl/blob/master/.github/workflows/format_check.yml. You have a .JuliaFormatter.toml` file, but this job will check that the code is formatted correctly.

I can give you a hand with this if you have questions 😄