Make CustomClipper extend Listenable

[pingbird]

Description

Currently CustomClipper does not allow outside libraries to listen to the repaint listenable provided to its constructor.

This PR fixes that by making CustomClipper extend Listenable just like CustomPainter.

Related Issues

#55992

Tests

I added the following tests:

• clip_test.dart/CustomClipper reclips when notified

Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I signed the [CLA].
• I read and followed the [Flutter Style Guide], including [Features we expect every widget to implement].
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I updated/added relevant documentation (doc comments with ///).
• All existing and new tests are passing.
• The analyzer (flutter analyze --flutter-repo) does not report any problems on my PR.
• I am willing to follow-up on review comments in a timely manner.

Breaking Change

Because this change adds methods, it could break code that implements CustomClipper instead of extending it.

Did any tests fail when you ran them?

• No, no existing tests failed, so this is not a breaking change.
• Yes, this is a breaking change. If not, delete the remainder of this section.

[goderbauer]

LGTM