Fix FP S6759 (prefer-read-only-props): Restrict the scope to functional components

[yassin-kammoun-sonarsource]

As reported here, S6759 (currently borrowing the implementation of prefer-read-only-props) raises false positives on React class components. The scope of the rule should be restricted to React functional components only. The rule should report on such components that neither use TypeScript's utility type Readonly nor pure read-only props interfaces/type aliases. The corresponding RSPEC should be updated accordingly.