Add support for font variation based theming to `Icon`

[guidezpl]

Followup to https://github.com/flutter/flutter/pull/106896.

https://github.com/flutter/flutter/issues/102560

Pre-launch Checklist

• [x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
• [x] I read the Tree Hygiene wiki page, which explains my responsibilities.
• [x] I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• [x] I signed the CLA.
• [x] I listed at least one issue that this PR fixes in the description above.
• [x] I updated/added relevant documentation (doc comments with ///).
• [x] I added new tests to check the change I am making, or this PR is test-exempt.
• [x] All existing and new tests are passing.

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[flutter-dashboard[bot]]

Golden file changes have been found for this pull request. Click here to view and triage (e.g. because this is an intentional change).

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Changes reported for pull request #109140 at sha 105c4171ce868480b4f39b7d0fb4743abe892d97

[flutter-dashboard[bot]]

Golden file changes are available for triage from new commit, Click here to view.

For more guidance, visit Writing a golden file test for package:flutter.

Reviewers: Read the Tree Hygiene page and make sure this patch meets those guidelines before LGTMing.

Changes reported for pull request #109140 at sha 23c40118aae7782251db4255b7cd1bd25313cdc8