Fix: LineHeight button to use MenuAnchor

[AtlasAutocode]

Description

Fix: LineHeight button to use MenuAnchor. Modelled changes on that used for header toolbar button.
(There is no rush to merge this fix - whenever it is convenient.)

Related Issues

• Fix LineHeight toolbar button popup menu appears disconnected #1981

Additional notes

In my copious free time, I may look to see if font, size, header and lineheight buttons can be consolidated to use a common base class.

Checklist

• [ X ] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [ X ] I titled the PR using Conventional Commits.
• [ X ] I did not modify the CHANGELOG.md nor the package version in pubspec.yaml files.
• [ X ] All existing and new tests are passing.
• [ X ] I have run the commands in ./scripts/before_push.sh and it all passed successfully

Breaking Change

Does your PR require developers 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).
• [ X ] No, this is not a breaking change.

[CatHood0]

By now MenuAnchor at least with LineHeight does not produce this error where appears below keyboard (at least as far as I tested it), so there should be nothing wrong with this change.

Thank you for taking the time to resolve this issue!.