[ruby-core:96385] [Ruby master Feature#16441] Enumerable#take_while_after

Issue #16441 has been reported by zverok (Victor Shepelev).

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Feature #16441: Enumerable#take_while_after
https://bugs.ruby-lang.org/issues/16441

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
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The method is just like `#take_while`, but also includes the item where condition became true.

Examples of usefulness:
```ruby
str = <<DOC
prologue
<<
1
2
3
>>
epilogue
DOC
# Imagine we want to take everything starting from << to >> in short and clean Ruby
# Surprisingly, our best guess would be infamous flip-flop:
p str.each_line(chomp: true).filter_map { _1 if _1 == '<<'.._1 == '>>' }
# => ["<<", "1", "2", "3", ">>"]

# Trying to achieve this with Enumerator, you _almost_ can express it, but...
p str.each_line(chomp: true).drop_while { _1 != '<<' }.take_while { _1 != '>>' }
# => ["<<", "1", "2", "3"] -- the last line is lost.

# So, Enumerable leaves us with this (which is harder to read, due to additional `.first`):
p str.each_line(chomp: true).drop_while { _1 != '<<' }.slice_after { _1 == '>>' }.first
# => ["<<", "1", "2", "3", ">>"]

# With proposed method:
p str.each_line(chomp: true).drop_while { _1 != '<<' }.take_while_after { _1 != '>>' }
# => ["<<", "1", "2", "3", ">>"]
```

The idea is the same as with flip-flops `..` vs `...` (sometimes we need to include the last element matching the condition, sometimes don't), and `while ... end` vs `do ... while`. Another example (from `Enumerator.produce` [proposal](https://bugs.ruby-lang.org/issues/14781)):

```ruby
require 'strscan'
scanner = StringScanner.new('7+38/6')
p Enumerator.produce { scanner.scan(%r{\d+|[-+*/]}) }.take_while { !scanner.eos? } # expresses meaning, but loses last element
# => ["7", "+", "38", "/"]
p Enumerator.generate { scanner.scan(%r{\d+|[-+*/]}) }.slice_after { scanner.eos? }.first # slice_after {}.first again
# => ["7", "+", "38", "/", "6"]
p Enumerator.produce { scanner.scan(%r{\d+|[-+*/]}) }.take_while_after { !scanner.eos? }
# => ["7", "+", "38", "/", "6"]
```

PS: Not sure about the name, suggestions are welcome



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