[php-src] Issue #24624: Feature request: native async/await syntax built on Fibers

Issue: https://github.com/php/php-src/issues/24624 Author: Eng3bbas ### Description PHP 8.1 added Fibers, but writing concurrent code still means picking a userland library (Amp, ReactPHP, Revolt) and its own Promise/Future types. This issue proposes native async/await syntax built on top of Fibers, so the engine can express concurrent code directly: ```php async function fetch(string $name, float $secs): string { await Async\delay($secs); return "$name done"; } $a = fetch("a", 0.2); // starts immediately and returns an Async\Future $b = fetch("b", 0.2); echo await $a, await $b; // ~0.2s total ``` A working prototype is in draft PR #24623. In short: - async (functions, methods, closures, arrow functions) is a contextual keyword, so existing async() functions keep working. - await is a new reserved keyword, still allowed as a member name. - Both are desugared in the parser into calls to a small Fiber-based scheduler (Async\Future, Async\async(), Async\wait(), Async\delay(), Async\suspend()). No new opcodes are needed. Open questions for discussion: - whether await as a reserved keyword is an acceptable BC break - how this should relate to the separate True Async RFC - what I/O / event-loop integration should look like (the prototype only has timers and cooperative yielding) I understand a change like this needs an RFC and discussion on the internals list. This issue and the draft PR are meant to make that discussion concrete.