[ruby-core:104410] [Ruby master Feature#18008] `keyword_init?` method for Struct

Issue #18008 has been reported by hkdnet (Ko Sato).

----------------------------------------
Feature #18008: `keyword_init?` method for Struct
https://bugs.ruby-lang.org/issues/18008

* Author: hkdnet (Ko Sato)
* Status: Open
* Priority: Normal
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I'd like to know whether my struct was initialized with `keyword_init: true` or not.
This information is useful when writing a deserializer (attached an example below).

```ruby
S1 = Struct.new(:a, :b, keyword_init: true)
S2 = Struct.new(:a, :b)

# Specialized for Struct
def serialize(d)
  d.to_h.merge(__class_name: d.class.name)
end

def deserialize(h)
  klass = Object.const_get(h.delete(:__class_name))
  if keyword_init?(klass)
    # If the class is created with keyword_init: true, the parameter should be passed as keywords
    klass.new(**h)
  else
    # Otherwise, each values are passed in the order of members.
    klass.new(*klass.members.map { |sym| h[sym] })
  end
end

def keyword_init?(klass)
  # I don't want to do this...
  # klass.keyword_init? looks cool. 
  klass.inspect.end_with?('(keyword_init: true)')
end

s1 = S1.new(a: 1, b: 2)

p s1
p s1_ = deserialize(serialize(s1))
p s1 == s1_

s2 = S2.new(1, 2)

p s2
p s2_ = deserialize(serialize(s2))
p s2 == s2_
```




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