[ruby-core:115952] [Ruby master Bug#20102] Introduce `Fiber#resuming?`

Issue #20102 has been reported by ioquatix (Samuel Williams).

----------------------------------------
Bug #20102: Introduce `Fiber#resuming?`
https://bugs.ruby-lang.org/issues/20102

* Author: ioquatix (Samuel Williams)
* Status: Open
* Priority: Normal
* Assignee: ioquatix (Samuel Williams)
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
There are some tricky edge cases when using `Fibre#raise` and `Fiber#kill`, e.g.

```ruby
fiber = nil

killer = Fiber.new do
	fiber.raise("Stop")
end

fiber = Fiber.new do
	killer.resume
end

fiber.resume
# 4:in `raise': attempt to raise a resuming fiber (FiberError)
# 4:in `block in <main>'
```

Async has to deal with this edge case explicitly by rescuing the exception:

https://github.com/socketry/async/blob/ffd019d9c1d547926a28fe8f36bf7bfe91d8a168/lib/async/task.rb#L226-L233

I'd like to avoid doing that and instead just ask "Can I kill/raise on this fiber right now?" which is determined by whether the fiber itself can be resumed or transferred to.

To address this, I'd like to introduce `Fiber#resuming?`:

```c
/*
 *  call-seq: fiber.resumed? -> true or false
 *
 *  Whether the fiber is currently resumed.
 */
VALUE
rb_fiber_resuming_p(VALUE fiber_value)
{
    struct rb_fiber_struct *fiber = fiber_ptr(fiber_value);

    if (FIBER_TERMINATED_P(fiber)) return RUBY_Qfalse;

    return RBOOL(fiber->resuming_fiber);
}
```

See the PR: https://github.com/ruby/ruby/pull/9382



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