feat(battery_plus): Introduce not_charging state on Linux

[vbuberen]

Description

Feature similar to #2399 and #2275

Had hard time trying to find out what is pending discharge state and made an assumption that it is similar to discharging as it was done here, for example: https://yarp.it/git-master/upowerBattery_8cpp_source.html

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.