Add support for setting the heading level for web semantics (#97894)

[victorgalo]

This change adds a new property in Semantics widget that would take an integer 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:
#97894

Engine part:
flutter/engine#52747

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], including [Features we expect every widget to implement].
• I signed the [CLA].
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is [test-exempt].
• All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on [Discord].