Windows support

- Swift publishes official toolchains for Windows [here](https://swift.org/download/) since Swift 5.3 ([blog post](https://swift.org/blog/swift-on-windows/)).
- Bazel also [supports Windows](https://docs.bazel.build/versions/4.1.0/install-windows.html) (I believe it always has).
- The Swift plugin for CLion 2021.1 also supports Windows ([blog post](https://blog.jetbrains.com/objc/2021/03/swift-on-windows-in-clion/)).
- The [Bazel plugin for IntelliJ](https://github.com/bazelbuild/intellij) also supports CLion.

Therefore there appears to be potential now for a Swift/Bazel/CLion developer environment on Windows.

- Is there any intent for `rules_swift` to support Windows as a host and target?
- If someone were to pursue this, where should they start?

The issue names no repository files, tests, or entry points. Start by reviewing the linked Swift Windows toolchain and Bazel Windows documentation, then inspect rules_swift's platform assumptions and determine whether host and target support are separate scopes. Done requires an agreed implementation plan and explicit Windows host/target acceptance criteria.

Written by the indexing model from the issue text.