[ruby-core:111264] [Ruby master Feature#19197] Add Exception#root_cause

Issue #19197 has been reported by AMomchilov (Alexander Momchilov).

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Feature #19197: Add Exception#root_cause
https://bugs.ruby-lang.org/issues/19197

* Author: AMomchilov (Alexander Momchilov)
* Status: Open
* Priority: Normal
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## Description

I would like to add a `#root_cause` method to `Exception`.

It returns the last exception in linked-list of causality chain, that is, the original exception (whose own `cause` is `nil`).

## Example

```ruby
e = begin
    raise 'a' # This is the root cause
  rescue => a
    begin
      raise 'b'
    rescue => b
      begin
        raise 'c' # This is the outermost cause assigned to `e`
      rescue => c
        c
      end
    end
  end
  
p(e)                   # => #<RuntimeError: c>
p(e.cause)             # => #<RuntimeError: b>
p(e.cause.cause)       # => #<RuntimeError: a>
p(e.cause.cause.cause) # => nil

p(e.root_cause)        # => #<RuntimeError: a>
p(e.root_cause.cause)  # => nil
```

# Motivation

There are some kinds of exceptions that can occur all over the place (and might be wrapped by arbitrarily many middlemen), but are attributable to a singular global cause. For example, a database outage could raise exceptions in almost every line of business logic of an app that uses ActiveRecord models.

Fundamentally, you wouldn't want an error report for every one of these lines. You'd want to look at the root cause, and bucket all SQL-connection issues into a single report, regardless of where they surface.

### Implementation

Draft PR: https://github.com/ruby/ruby/pull/6913



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