feat: Implement extension methods (RFC prototype, phase 1)

[hollyschilling]

Adds Swift-style extension blocks that declare additional methods for an existing class or interface, resolved at call time only when the receiver does not define the method itself:

extension Target {
    public function helper(): string { /* $this is the receiver */ }
}

• extension is a contextual keyword (T_EXTENSION), lexed only when followed by another identifier, mirroring the enum technique; a extension extends/implements carve-out preserves classes named "extension", and T_EXTENSION is semi-reserved.
• Extension blocks compile to a synthetic final anonymous class; a new ZEND_BIND_EXTENSION opcode (212) registers its methods, keyed by the lowercased resolved target name, when the declaration executes (load-gated, like function declarations).
• zend_std_get_method gains one fallback on its miss path: real method, then __call trampoline, then extension registry lookup (most-derived target first: inheritance chain, then interfaces). Registered methods are flagged ZEND_ACC_NEVER_CACHE so the polymorphic inline cache and JIT are bypassed in this prototype.
• Methods only: properties, constants, and cases are compile errors.
• Indirect invocation (callables, reflection, method_exists) deliberately does not see extensions.

PROTOTYPE LIMITATIONS: the registry is a process global, so user-class method pointers dangle after request 1 in multi-request SAPIs (CLI only until it moves into zend_executor_globals); duplicate registrations are silently ignored (first wins).