Add support for platform windows

[cjlarose]

Fixes #57

Several gems declare platform support for windows, including several dependencies of a newly-generated rails app

• debug (1.9.1)
• io-console (0.7.2)
• irb (1.11.1)
• psych (5.1.2)
• rdoc (6.6.2)
• reline (0.4.2)
• stringio (3.1.0)

[cjlarose]

Wasn't 100% sure what engines to list here, but basically took the list from Gem::Platform::WINDOWS as defined here

https://github.com/rubygems/rubygems/blob/8c98dbf1b1f6005b4e0343999d7fd556428fa0c2/bundler/lib/bundler/rubygems_ext.rb#L208

[jonknapp]

@inscapist would we be able to get this merged into your fork?

[jonknapp]

For context it looks like “windows” was added as an alias for existing windows platforms in bundler: rubygems/rubygems#5650

I encountered the issue as new Rails app Gemfiles use this alias instead of the mingX platforms it did before.

[inscapist]

@jonknapp thanks for providing the context, a PR has been merged into my fork!

[robbevp]

@manveru Any chance you could take a look at this? If a gem like irb gets updated now, this results in an unclear error /nix/store/np0gx2m7xkxrvx3ack88bv1zlpkcrgw9-bundix-2.5.2/lib/ruby/gems/3.1.0/gems/bundix-2.5.0/lib/bundix/nixer.rb:100:in serialize': Cannot convert to nix: nil`

I tried these changes in a repository where I ran into this issue and they generated the expected gemset.