[widgets/shortcuts] Add includeSemantics property to the Shortcuts widget.

[davidhicks980]

Adds an includeSemantics property to the Shortcuts widget.

@gspencergoog and I discussed this change to make introducing Shortcuts easier on the MenuAnchor widget. This change should also make testing semantics trees slightly less onerous (assuming this doesn't harm accessibility, that is).

On the point of accessibility, I'm not sure what useful semantic information is exposed by the Shortcuts widget's semantic node, since it's basically just an unfocusable keyboard listener. However, I kept the includeSemantics defaulting to true so as to not break semantics tests in the Flutter repo.

Fixes #152076

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.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.

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[gspencergoog]

[justinmc]

LGTM 👍

[auto-submit]

auto label is removed for flutter/flutter/152077, due to - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.