feat: add windows, macos, linux and unix to targets

[wolfv]

I added a few more targets that should make it easier to select packages for some sets of platforms.

You can now do:

[target.unix.dependencies]
special-unix-pkg = "*"

[target.windows.dependencies]
pywin32 = "3.12"

# ... linux and macos are also available.

We could consider to overload osx, too. The correct name for osx is macOS though, since a while. Unfortunately that is not reflected in the "subdir" name.

[ruben-arts]

This looks good, If you add some tests and documentation we can merge it!

[ruben-arts]

First thing i tried was pixi add --platform unix pytest which doesn't use this target selector. Might be nice to use that there aswell.

[wolfv]

It takes me more than 5 minutes to get this done, so let's merge & file a new issue?