feat(battery_plus): Add battery save mode check on MacOS

[vbuberen]

Description

While testing SPM for battery_plus was surprised that the check for battery save mode throws Unimplemented.... on MacOS, because such check is available in MacOS since version 12.0: https://developer.apple.com/documentation/foundation/processinfo/1617047-islowpowermodeenabled
This PR adds implementation for this check and it seems to work fine:

Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I titled the PR using Conventional Commits.
• I did not modify the CHANGELOG.md nor the plugin version in pubspec.yaml files.
• All existing and new tests are passing.
• The analyzer (flutter analyze) does not report any problems on my PR.

Breaking Change

Does your PR require plugin users to manually update their apps to accommodate your change?

• Yes, this is a breaking change (please indicate that with a ! in the title as explained in Conventional Commits).
• No, this is not a breaking change.

[miquelbeltran]

LGTM!