Ruby 4.0 introduces `Array#rfind`:
https://bugs.ruby-lang.org/issues/21678

This PR adds new `Style/ReverseFind` cop that identifies places
where `array.reverse.find` can be replaced by `array.rfind`.

## Safety

This cop is unsafe because it cannot be guaranteed that the receiver
is an `Array` or responds to the replacement method.

## Example

```ruby
# bad
array.reverse.find { |item| item.even? }
array.reverse.detect { |item| item.even? }
array.reverse_each.find { |item| item.even? }
array.reverse_each.detect { |item| item.even? }

# good
array.rfind { |item| item.even? }
```

## Additional Information

Using this approach improves performance as follows:

```ruby
require 'benchmark/ips'

Benchmark.ips do |x|
  array = *(1..100)

  x.report('reverse.find') { array.reverse.find { it == 42 } }
  x.report('reverse_each.find') { array.reverse_each.find { it == 42 } }
  x.report('rfind') { array.rfind { it == 42 } }

  x.compare!
end
```

```console
$ ruby rfind.rb
ruby 4.0.0 (2025-12-25 revision 553f1675f3) +PRISM [arm64-darwin24]
Warming up --------------------------------------
        reverse.find    81.778k i/100ms
   reverse_each.find    59.397k i/100ms
               rfind    89.719k i/100ms
Calculating -------------------------------------
        reverse.find    813.856k (± 0.7%) i/s -      4.089M in   5.024325s
   reverse_each.find    596.506k (± 0.4%) i/s -      3.029M in   5.078403s
               rfind    889.829k (± 0.5%) i/s -      4.486M in   5.041482s

Comparison:
               rfind:   889828.8 i/s
        reverse.find:   813855.8 i/s - 1.09x  slower
   reverse_each.find:   596505.7 i/s - 1.49x  slower
```

This cop is placed under `Style` rather than `Performance` because it encourages
using the dedicated `rfind` method, which is considered a matter of style.
As a result, the code becomes shorter and requires fewer method calls,
resulting in a simpler style.