This change adds a new property in Semantics widget that would take an integer value corresponding to the heading levels defined by the ARIA heading role. This is necessary in order to get proper accessibility and usability in a website for users who rely on screen readers and other assistive technologies.

Issue fixed by this PR:
flutter/flutter#97894

Framework part:
flutter/flutter#148195

Pre-launch Checklist

• I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
• I read the [Tree Hygiene] wiki page, which explains my responsibilities.
• I read and followed the [Flutter Style Guide] and the [C++, Objective-C, Java style guides].
• I listed at least one issue that this PR fixes in the description above.
• I added new tests to check the change I am making or feature I am adding, or Hixie said the PR is test-exempt. See [testing the engine] for instructions on writing and running engine tests.
• I updated/added relevant documentation (doc comments with /// ).
• I signed the [CLA].
• All existing and new tests are passing.