Add Fantom.getHostPlatform() to expose test runner host OS (#56990)

Summary:
Pull Request resolved: https://github.com/facebook/react-native/pull/56990

Adds a new `Fantom.getHostPlatform()` API that returns the host
operating system where the Fantom test runner is running (e.g. 'linux',
'macos', 'windows', 'android'). This is different from React Native's
`Platform.OS`, which always reflects the React Native target platform
being tested.

The value is determined by the Node.js Jest runner (via
`process.platform`) and baked into the JS bundle through the existing
`setConstants(...)` pipeline, so no native changes are needed.

- Add `HostPlatform` type and `hostPlatform` field to
  `FantomRuntimeConstants`.
- Populate `hostPlatform` from the host platform in the entrypoint
  template (maps darwin -> macos, win32 -> windows, etc.).
- Expose `getHostPlatform()` from the public Fantom API.

Changelog: [Internal]

Reviewed By: javache

Differential Revision: D106669673

fbshipit-source-id: 6948e0c1eb0075131211d8c5099da96a87306b0a