Fix inherit_mode for third-party gems

fixes #1126

See palkan/action_policy#103 (comment)

If a third-party gem, e.g. `action_policy`, defines their
`config/default.yml` and supplements to the default RSpec DSL that we
provide in our `config/default.yml`, `rubocop`'s behaviour is to
actually override the configuration, as `inherit_mode` is `override` for
lists by default.

RuboCop has recently [fixed the problem with ignored
`inherit_mode`](rubocop/rubocop#9952) and we've
[bumped to use `rubocop` version that includes the
fix](rubocop/rubocop-rspec#1181), but we haven't
adjusted our `config/default.yml` to merge by default.

This is causing both user project RSpec DSL configuration and
third-party gem RSpec DSL configuration to override our default,
rendering our cops blind.

### Example

A new project

```ruby
# Gemfile
source 'https://rubygems.org'

gem 'action_policy'
gem 'rubocop-rspec'
```

```yml
# .rubocop.yml
require:
  - rubocop-rspec

inherit_gem:
  action_policy: config/rubocop-rspec.yml
```

```ruby
# spec/a_spec.rb
RSpec.describe 'A' do
  it_behaves_like 'a'
  it_behaves_like 'a'

  describe_rule :show? do
    succeed 'when post is published'
    succeed 'when post is published'
  end
end
```

Ideally, both the duplicated `it_behaves_like` and `succeed` should be
detected. However, `action_policy`'s `Includes/Examples` setting
overrides ours, and `it_behaves_like` disappears from this list. As a
result, `rubocop` only detects the duplication of `succeed`, but not of
`it_behaves_like`.

CHANGELOG.md

@@ -2,6 +2,8 @@
 
 ## Master (Unreleased)
 
+* Fix merging RSpec DSL configuration from third-party gems. ([@pirj][])
+
 ## 2.5.0 (2021-09-21)
 
 * Declare autocorrect as unsafe for `ExpectChange`. ([@francois-ferrandis][])

config/default.yml

@@ -5,7 +5,20 @@ RSpec:
     - "**/*_spec.rb"
     - "**/spec/**/*"
   Language:
+    inherit_mode:
+      merge:
+        - Expectations
+        - Helpers
+        - Hooks
+        - HookScopes
+        - Runners
+        - Subjects
     ExampleGroups:
+      inherit_mode:
+        merge:
+          - Regular
+          - Skipped
+          - Focused
       Regular:
         - describe
         - context
@@ -20,6 +33,12 @@ RSpec:
         - fcontext
         - ffeature
     Examples:
+      inherit_mode:
+        merge:
+          - Regular
+          - Skipped
+          - Focused
+          - Pending
       Regular:
         - it
         - specify
@@ -62,6 +81,10 @@ RSpec:
       - all
       - suite
     Includes:
+      inherit_mode:
+        merge:
+          - Examples
+          - Context
       Examples:
         - it_behaves_like
         - it_should_behave_like
@@ -73,6 +96,10 @@ RSpec:
       - to_not
       - not_to
     SharedGroups:
+      inherit_mode:
+        merge:
+          - Examples
+          - Context
       Examples:
         - shared_examples
         - shared_examples_for