Brings file_selector_macos into flutter/plugins from FDE, with the following changes:
- Refactored slightly to allow for unit tests of almost all of the native code
- Added native unit test coverage
- Translated to Swift, to follow repo conventions for macOS plugins
- Added an in-package example (almost an exact duplicate of the app-facing version, but written against the platform interface, as is our current practice)
- Moved to an in-package method channel. As part of that, moved the flattening of type groups from Swift to Dart.

Does not currently include native UI tests to allow for end-to-end testing (since Flutter integration tests can't be used). They should be added later (they are currently blocked on https://github.com/flutter/flutter/issues/90673), but the unit tests give substantial coverage, making it substantially better to move the plugin now to get those tests running.

macOS portion of https://github.com/flutter/flutter/issues/70221