[ruby-core:111818] [Ruby master Feature#19344] Regexp.new: stricter handling of second argument

Issue #19344 has been reported by zverok (Victor Shepelev).

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Feature #19344: Regexp.new: stricter handling of second argument
https://bugs.ruby-lang.org/issues/19344

* Author: zverok (Victor Shepelev)
* Status: Open
* Priority: Normal
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Since Ruby 3.2 (#18788), the second argument to `Regexp.new` can be:
1. Integer: then it is treated as a combination of `Regexp::<constant>` flags
2. String: then it is treated as a combination of string flags
3. `nil` or `false`: then it is ignored
4. **any other truthy value**: then it is treated as an "ignore case" option.

The fourth one is confusing, especially since the introduction of the flexibility of flags: one might erroneously assume or forget the protocol, and the naive check will strengthen the assumption:
```ruby
# maybe it accepts the array of string options?..
Regexp.new('foo', %w[i]) #=> /foo/i -- oh, seems it does

# would the symbol work?..
Regexp.new('foo', :i) #=> /foo/i -- awesome, it works!
```

I propose to change (4) to only handle literal `true` value, and raise `TypeError` on any unsupported type.

On compatibility: I believe that whenever the usage of boolean to distinguish "ignore case/respect case" is deliberate (like [in rubygems](https://github.com/rubygems/rubygems/blob/master/lib/rubygems/optparse/lib/optparse.rb#L442)), the code already passes true/false, not any random value. Otherwise, the change in Ruby 3.2 might also have broken it (what if it previously passed strings, meaning them to be "truthy values"?)

PS: BTW, the documentation for (4) was completely lost in Ruby 3.2, due to [this PR](https://github.com/ruby/ruby/pull/5815). cc @jeremyevans0, @burdettelamar 



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