Feature: Add Promise v3 template types for all public functions.
(#40 by @WyriHaximus and @clue)

All our public APIs now use Promise v3 template types to guide IDEs and static
analysis tools (like PHPStan), helping with proper type usage and improving
code quality:

assertType('bool', await(resolve(true)));
assertType('PromiseInterface<bool>', async(fn(): bool => true)());
assertType('PromiseInterface<bool>', coroutine(fn(): bool => true));

Feature: Full PHP 8.3 compatibility.
(#81 by @clue)

Update test suite to avoid unhandled promise rejections.
(#79 by @clue)