Check function literals in `unused-param`

[StephenButtolph]

Closes #965

See #965 for the motivation behind this PR.

• [X] Added tests
• [X] I believe this code follows the existing style, but if this could be improved I'm very open to any feedback
• [x] Passes CI

This does potentially significantly change the performance of this rule because we will now walk into the function bodies. If we are concerned about a performance regression here I think an alternative could be to either add a new rule or add a configuration to the rule to gate checking anonymous functions.

[chavacava]

Hi @StephenButtolph, thanks for the PR and the proposal for improving the rule. I'll try to review the PR in the coming days.

[chavacava]

merged, thanks @StephenButtolph