refactor: migrate avoid_returning_widgets rule

[andrew-bekhiet-solid]

feat: also check FunctionDeclarationStatement

[gemini-code-assist]

Code Review

This pull request refactors the AvoidReturningWidgetsRule to utilize the new analyzer-based rule APIs, introducing a dedicated AvoidReturningWidgetsVisitor and adding comprehensive unit tests. Additionally, it registers the rule in the main plugin entry point and cleans up unused dependencies in pubspec.yaml. A critical issue was identified in excluded_identifiers_list_parameter.dart where accessing classDeclaration.namePart will cause a compilation error because namePart does not exist on ClassDeclaration; a suggestion was provided to use classDeclaration.name.lexeme instead.

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