Add .bazelci/presubmit.yml

[j3parker]

Related to #42

I also need to submit an issue in their repo, according to the documentation.

I've kept this pretty basic. We'd like to test on the 3 major platforms. There are some other things I'd like in the future:

• run & test targets: blocked on #9
• buildifier linting: blocked on us formatting our code
• using remote builds (i.e. the rbe_ubuntu1604 platform): this is probably blocked on as-of-yet-unfiled issue where we have a bit too many implicit dependencies on the exec environment (e.g. libicu) it's why we aren't doing bazel build inside our GitHub action (which runs in the "official" Bazel containers, which RBE may use...)

For the extra stuff that might not succeed on attempt 1 it'll be easier to add those as follow-ups, once we can trigger CI :) My goal is to just get something basic working.

[j3parker]

I could of course make this more minimal but I'm feeling lucky ;)