Multiple fixes related to heading levels:
* Fix heading level absorption. Heading level would get erased when a semantics config is absorbed into another. With this change the highest heading level wins.
* Add `headingLevel` to the diagnostics of `SemanticsNode`.
* Add unit-tests for heading levels.
* Add an a11y use-case for headings.
Improves https://github.com/flutter/flutter/issues/46789 and general accessibility of headings.
