Update supported languages in Windows

[native-api]

https://travis-ci.community/t/support-language-generic-and-language-minimal-in-windows/6377

Checked support status at: https://travis-ci.org/native-api/test_travis/builds/625179816 (w/o config validation), https://travis-ci.org/native-api/test_travis/builds/625189419 (w/ config validation).

Note that I added comments about unsupported but accepted language:s to help keep track of them. If you think you can do without, feel free to remove them.

• C# wasn't documented
• There are some undocumented language:s in https://github.com/native-api/travis-build/blob/master/lib/travis/build/config.rb#L128 that are accepted but don't currently work. Are they needed there?
  • I reckon powershell is. But script probably isn't (unless you use it internally or something special; Build Config Validation doesn't recognize it).

Contrary to what I wrote in https://travis-ci.community/t/support-language-generic-and-language-minimal-in-windows/6377, language: minimal does NOT produce a VM. My fault. (Note to self: double check my claims before publishing them.)

• So an additional change in VM selection logic is needed for language: minimal to be considered supported. (Since this component is not open-source, I cannot help you here.)