[shared_preferences] Convert macOS to Pigeon

[stuartmorgan-g]

Converts the macOS implementation of shared_preferences to Pigeon now that we have a Swift generator.

• The Dart implementation is taken directly from the iOS package, which was already converted to Pigeon, just with the names changed.
• The Pigeon definition is also taken directly from the iOS package, with the PigeonOptions adjusted and some TODO comments added for future improvements.
• The Dart tests are mostly taken directly from the iOS package, but some missing type coverage has been added.
• The Swift implementation is just moving existing implementations into the new Pigeon API functions, with the arg extraction thrown away.
• The Swift tests are mostly straight conversion, but I noticed that remove was missing coverage, as were some types, so I added those.

Part of flutter/flutter#117914

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the relevant style guides and ran the auto-formatter. (Unlike the flutter/flutter repo, the flutter/plugins repo does use dart format.)
• I signed the CLA.
• The title of the PR starts with the name of the plugin surrounded by square brackets, e.g. [shared_preferences]
• I listed at least one issue that this PR fixes in the description above.
• I updated pubspec.yaml with an appropriate new version according to the pub versioning philosophy, or this PR is exempt from version changes.
• I updated CHANGELOG.md to add a description of the change, following repository CHANGELOG style.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

[stuartmorgan-g]

I'll need to fix flutter/flutter#117994 so that lint_podspec passes; that doesn't need to block review though since it'll only change generated code.

[gaaclarke]

lgtm!