* Ractor.yield no longer exists
* Ractor.shareable_proc returns a copy of the given proc
* Improve wording for monitoring/unmonitoring ports

Although the Ractor API is still experimental and may change, and there
may be some implementation issues, we should no longer say that there
are many.

Hopefully we can remove this warning entirely for Ruby 4.1

```ruby
$VERBOSE = true

Ractor.store_if_absent :key do
end #=> warning: the block passed to 'Ractor.store_if_absent' defined at <internal:ractor>:474 may be ignored
```

call-seq:
  Ractor.sharable_proc(self: nil){} -> sharable proc

It returns shareable Proc object. The Proc object is
shareable and the self in a block will be replaced with
the value passed via `self:` keyword.

In a shareable Proc, the outer variables should
* (1) refer shareable objects
* (2) be not be overwritten

```ruby
  a = 42
  Ractor.shareable_proc{ p a }
  #=> OK

  b = 43
  Ractor.shareable_proc{ p b; b = 44 }
  #=> Ractor::IsolationError because 'b' is reassigned in the block.

  c = 44
  Ractor.shareable_proc{ p c }
  #=> Ractor::IsolationError because 'c' will be reassigned outside of the block.
  c = 45

  d = 45
  d = 46 if cond
  Ractor.shareable_proc{ p d }
  #=> Ractor::IsolationError because 'd' was reassigned outside of the block.
```

The last `d`'s case can be relaxed in a future version.

The above check will be done in a static analysis at compile time,
so the reflection feature such as `Binding#local_varaible_set`
can not be detected.

```ruby
  e = 42
  shpr = Ractor.shareable_proc{ p e } #=> OK
  binding.local_variable_set(:e, 43)
  shpr.call #=> 42 (returns captured timing value)
```

Ractor.sharaeble_lambda is also introduced.
[Feature #21550]
[Feature #21557]

[Feature #21262]