6.x: Add stricter naming convention sniffs

[dereuromark]

Summary

This PR introduces stricter naming convention enforcement for CakePHP 6.x, aligned with PSR naming conventions.

New Sniffs

ValidPropertyNameSniff (new)

• Flags underscore-prefixed properties ($_propertyName)
• Applies to all visibility levels (public, protected, private)

ValidFunctionNameSniff (updated)

• Now flags underscore-prefixed protected/private methods
• Exception: Entity accessor/mutator patterns are still allowed (_getFieldName(), _setFieldName())
• Added T_ENUM support
• Fixed $_magicMethods property naming (was using underscore prefix itself)

Ruleset Changes

• Removed PSR2.Classes.PropertyDeclaration.Underscore exclusion - PSR2 property underscore checks now apply
• Added new Slevomat sniffs for modern PHP:
  • RequireNullCoalesceEqualOperator - Require ??= operator
  • RequireNullSafeObjectOperator - Require ?-> operator
  • RequireSelfReference - Require self:: instead of class name
  • NullTypeHintOnLastPosition - Enforce string|null instead of null|string

Migration

Projects upgrading to CakePHP 6.x should:

1. Rename underscore-prefixed properties: $_propertyName → $propertyName
2. Rename underscore-prefixed methods (except Entity accessors): _methodName() → methodName()

The cakephp/upgrade tool has Rector rules to automate these renames.

Tests

All existing tests pass, new test cases added for:

• Property underscore detection (class and trait)
• Method underscore detection with Entity accessor exception

[dereuromark]

You're right - removed the custom ValidPropertyNameSniff. The PSR2 built-in handles properties.

Kept the custom ValidFunctionNameSniff though, since it allows the Entity accessor/mutator pattern (_getName, _setField) which the PSR2 method sniff would flag.