Platform-specific overrides

[TravisSpomer]

We need a way to have tokens that are only exported for specific platforms, and for some tokens to have different values on each platform. We'd want to use this sparingly, but it's going to be unavoidable in some cases. e.g.:

• There are platform-specific concepts like the Apple SegmentedControl (basically, their tabstrip) that we might want to restyle on Apple platforms, but not even export for Windows and CSS, since that control doesn't exist elsewhere.
• At the lower end of the corner radii spectrum we're consistent across platforms, but we want to slightly modify the larger radii on some platforms to fit better with the platform's visual style.

This would probably work just by adding some kind of platform node to certain tokens that would signify that all platforms other than the specified ones should ignore that node and its children. There could also be a way to specify the reverse, such as exceptPlatform. For example:

"SegmentedControl" : {
  "platform": [ "mac", "ios" ],
  "Root": {
    "Fill": { /* ... */ }
  }
  /* ... */
}

[TravisSpomer]

This is nearly finished in the platform-overrides branch.

• [ ] Currently in that branch, it uses the WinUI overrides for all platforms; I need to make it actually platform-specific
  • [ ] This is yet another thing that's not possible with a standard Style Dictionary config, so I'll first need to change how we build the platform outputs
• [ ] winui is the only supported platform override; I need to add others
• [ ] I didn't build a way to exclude a set of tokens from the output for a specific platform; I'll need to open a new issue to track that in case it's needed
• [ ] Update documentation

[TravisSpomer]

This is now working as of this commit. I still need to write documentation for it.