[Wildcard Variables] Linter Support

[pq]

A meta-issue to discuss and track work on linter support for wildcard variables.

• Wildcards Feature Spec

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Existing Lints

• [ ] https://github.com/dart-lang/linter/issues/4968
• [ ] https://github.com/dart-lang/linter/issues/4974
• [x] #4995

New Lints

• [ ] #4972
• [x] ~https://github.com/dart-lang/linter/issues/4971~