
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.