add ContaintsStmts interface to mark stmt classes with nested stmts inside

[TomasVotruba]

This PR is revision #836 to address only mentioned issue with the ClassMethod.
It's the only node, that can have $stmts = null value. Such method can be abstract or part of Interface_, so there is nothing to iterate on. This revision exclude the ClassMethod

This would solve run custom Rector tests on PHPUnit 12, remove lot of vendor patching in Rector /vendor (ugly hack), make make writing PHPStan rules for all nodes that have $stmts easier and streamline working with the $stmts property in userland

[samsonasik]

@TomasVotruba just for note: we have custom node: FileWithoutNamespace that is StmtsAwareInterface instance, so that need update, if this is accepted, we need to have keep compat of StmtsAwareInterface to keep working without bc break before a major version as possible, like class_alias() usage.