Generic/NestingLevel: support anonymous functions

[rodrigoprimo]

Is your feature request related to a problem?

Similar to #742, while working on improving code coverage for Generic.Metrics.NestingLevel, I noticed that the sniff doesn't check the nesting level of anonymous functions, and I believe it should.

• I have read the Contribution Guidelines and this is not a support question.
• I intend to create a pull request to implement this feature.

[jrfnl]

Makes sense to me (and once support for PHP 8.4 property hooks gets added, those should probably also be checked).

Note: checking closures declared via arrow functions probably doesn't make sense as those can't contain more than one statement.