Add experimental list<T> type declarations

[tony1877]

This adds an experimental list type declaration prototype.

Implemented:

• parser support for list
• runtime validation for parameters, return types, and properties
• list-shape validation using zend_array_is_list()
• element type validation
• nullable ?list
• Reflection string rendering as list
• return covariance with array
• parameter contravariance with array
• explicit rejection of list inside union types for now

Tests run:

• sapi/cli/php run-tests.php Zend/tests/type_declarations/list_type_*.phpt
  • 14 passed, 0 failed
• sapi/cli/php run-tests.php Zend/tests/type_declarations
  • 503 passed, 7 skipped, 0 failed
• sapi/cli/php run-tests.php ext/reflection
  • 505 passed, 13 skipped, 0 failed
• sapi/cli/php run-tests.php Zend/tests/type_declarations/typed_properties_*.phpt
  • 119 passed, 1 skipped, 0 failed
• sapi/cli/php run-tests.php Zend/tests/type_declarations/variance Zend/tests/type_declarations/union_types/variance Zend/tests/type_declarations/intersection_types/variance
  • 78 passed, 0 failed

Notes:

• This is a prototype and intentionally keeps union support out of scope because the existing union type machinery assumes complex list payloads are union/intersection/class-name shaped.