feat(connectivity_plus): add checkConnectivity.hasConnectivity() utility getter

[EchoEllet]

Description

A very minor feature that is commonly used (e.g., serverpod_flutter):

final result = await checkConnectivity();
print(result.hasConnectivity);

I have also fixed a typo in Connectivity.onConnectivityChanged and extracted duplicated doc comments between onConnectivityChanged and checkConnectivity using a template named no_connectivity_case.

Related Issues

• Fix [Request]: Add hasConnectivity getter extension to List<ConnectivityResult> #3841

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.

[EchoEllet]

This PR does not modify sensors_plus, so this CI failure is most likely irrelevant.