ci: add macOS and Windows to build matrix

[myitcv]

Per discussion with @leitzler, we should add macOS to the build matrix in order to increase confidence for our macOS users. We're blind on that right now, and things like file watching (see #477) is OS-specific and sensitive.

• likely want to keep Docker images for Linux builds; these serve as a useful means of quickly re-running tests without requiring the user to have Vim/Gvim build locally (https://github.com/myitcv/govim/wiki/govim-tests#running-tests---inside-docker)
• likely want to move to GitHub actions after we have a working Travis setup with macOS. Less moving parts that way, slightly more mature setup on Travis, Travis macOS builders are quite fast. Again, we'd want to keep the Linux Docker approach even when we move to GitHub actions, in order that we dog-food that setup
• you cannot have a macOS image (wrong type of kernel?) hence we would need to resort to building Vim, installing the right Go version for each macOS CI run. Not critical, but not ideal either. Perhaps not so much of an issue if we consider the macOS build matrix entries as more of a fail-safe (could for example configure them to only run on master)

[myitcv]

As part of using GitHub actions we should use @rogpeppe's work on CUE schemas:

https://github.com/heetch/cue-schema/tree/master/github/actions

[myitcv]

In 59ef3d6 we moved to GitHub actions. Now we should really add macOS and Windows. But they don't need to be full-blown suites like on Linux - that can (and should) remain Docker-based.

Just something that:

• installs Vim (https://github.com/marketplace/actions/setup-vim)
• installs Go
• runs (race) tests

[myitcv]

For Windows support we will need to investigate something like https://pkg.go.dev/github.com/containerd/console?tab=doc as an alternative to our current PTY package.