Add a GitHub Actions CI pipeline

[MisterDA]

Network is currently failing?
Close #7.

[Alizter]

Let's disable the windows build for now, since we expect it to fail. We can reenable it when we add windows support.

[Alizter]

Lwt is an optional dependency which means notty.lwt won't even build so examples/ won't find it.

[Alizter]

We should mark lwt as a test dependency in dune-project like (lwt :with-test).

[Alizter]

This relies on the @default alias which is by default @all. We don't want to build everything in benchmarks/ for instance. To do this have a look at https://dune.readthedocs.io/en/stable/reference/aliases/default.html

We should probably just build src, src-unix and src-lwt for @default.

That is however orthogonal to what we want to do here. We are probably only interested in doing dune build @install which builds an install layout. Afterwards we do dune runtest which is just dune build @runtest.