Add migration script for upgrading from 2.x to 3.x

[Neluxx]

The upgrade from 2.x to 3.x renames most of the trait methods and changes the behaviour of some of them.
Doing this by hand is tedious and error-prone in larger test suites.

We should add a migrate.php script to the package root that follows docs/Migration.md.

The script should:

• renames calls of the old methods to the new assert* methods (testDataValidation* → assertValidation*)
• replaces the removed testRules() with assertRules() and lists those calls for manual review, since the behaviour differs
• lists calls of rule-dedicated methods, which now only check their own rule
• lists old method names it couldn't replace safely (strings, callables, etc.)
• optionally updates the package's version constraint in composer.json