Add windows build to github actions

[jefferis]

We have not previously had windows on CI. As @dokato noted, it seems that the stumbling block for windows was CMTK. Options

1. add windows without CMTK
2. install using cygwin action

I think it would be worth doing 1 even if we plan to do 2.

[dokato]

Re: 1, I actually tried with omitting the step that installs CMTK, but then the check on windows failed:
https://github.com/dokato/nat/runs/3030019476?check_suite_focus=true#step:9:713

Is there some flag/option that one needs to set before check to avoid running CMTK tests?

[jefferis]

Ah thanks. That problem is actually not to do with CMTK directly, but a secondary issue on windows. I do some tests to make sure that cmtk.bindir behaves by setting up a fake cmtk install

https://github.com/natverse/nat/blob/master/tests/testthat/test-cmtk.R#L29

the problem there is something to do with the behaviour of normalizePath on Windows. Likely a mix of forward and reverse slashes.