Add configurable naming for variadic arguments (opt-out) and related tests/docs

[savinmikhail]

Motivation

• Allow the rector to convert positional arguments that map to a variadic parameter into deterministic named arguments like name1, name2 while keeping the feature configurable.
• Preserve existing behavior by making variadic naming opt-out and avoid surprising changes for callers that rely on the previous behavior.
• Document the design and testing strategy for variadics to clarify behavior and edge cases.

Description

• Added a new configuration key ALLOW_NAMED_VARIADIC_ARGUMENTS to AddNamedArgumentsRector and expose configuration indices via STRATEGY and ALLOW_NAMED_VARIADIC_ARGUMENTS, with the variadic option defaulting to enabled.
• Implemented variadic handling by mapping arguments beyond the last declared parameter to the last parameter when it is variadic, and naming each variadic item deterministically as <paramName>1, <paramName>2, etc., using internal counters.
• Introduced resolveParameterForArgumentIndex() helper in both AddNamedArgumentsRector and DefaultStrategy to unify argument-to-parameter mapping, and added hasVariadicArguments() to early-skip refactoring when the variadic option is disabled.
• Updated configure() parsing and validation to accept the new boolean variadic option and to preserve existing strategy configuration, added README clarification for the new option, and added docs/issue-15-implementation-plan.md describing the approach.

Testing

• Added unit tests and fixtures: tests/VariadicDisabledStrategy/AddNamedArgumentsRectorTest.php with tests/VariadicDisabledStrategy/Fixture/variadic_disabled.php.inc, and tests/DefaultStrategy/Fixture/variadic.php.inc and variadic_mixed_signature.php.inc were updated/added to assert variadic naming behavior.
• Ran the project's PHPUnit test suite (vendor/bin/phpunit) including the new and updated fixtures, and all tests passed.