This patch extends the `FeatureTest.platform` getter to provide the necessary information, and uses that one to implement a Fluent `PLATFORM` function based on: https://searchfox.org/mozilla-central/rev/d1fbe983fb7720f0a4aca0e748817af11c1a374e/intl/l10n/rust/fluent-ffi/src/bundle.rs#81-98