refactor: move platform detection to Rake module, and delete Rake::Win32 module

[pvdb]

The Rake::Win32 module existed solely to answer one question: are we running on Windows?

That single-method module was more ceremony than it was worth, and it meant platform detection was split across two places — Win32.windows? in win32.rb and Application#unix? in application.rb.

Both methods now live together as Rake.windows? and Rake.unix? on the Rake module itself, which is the natural home for gem-level helpers.

Application#windows? and Application#unix? are kept as thin delegators so nothing calling them through an application instance breaks.

Beyond tidying up the code, the refactoring also untangles two distinct uses of platform detection that were previously conflated. 🎉

The first is a static, load-time concern: deciding which tests to define or which assertions to run. This only needs a simple module-level predicate — call it once when the class loads and you're done.

The second is a dynamic, instance-level concern: branching on platform inside Application methods, and — critically — being stubbable in tests so that platform-specific code paths can be exercised regardless of where CI runs.

These two uses have different requirements.

1. The new Rake.windows? / Rake.unix? module methods serve the first cleanly.
2. The delegators on Application preserve the second — including the ability to stub @app.windows? / @app.unix? in tests, which several existing tests rely on.

I hope you like it, @hsbt 🤞 - this isn't just cleanup, it's a separation of concerns that makes the intent of each call site
clearer.

[pvdb]

This preserves the original TestRakeApplication.test_windows exclusivity check, but - arguably - with a more intent-revealing name, and also by documenting it (as the original assertion was somewhat esoteric) 😃