Support Ruby 2.7's pattern matching for Layout/SpaceAroundKeyword

This PR supports guard `if` and `unless` syntax keywords of Ruby 2.7's
pattern matching for `Layout/SpaceAroundKeyword`.

```ruby
case 42
in 42 if true
  puts 'hello'
end
```

These nodes are called `if-guard` and `unless-guard` when using guard `if` and `unless`
in pattern matching.

```ruby
% ruby-parse example.rb
(case-match
  (int 42)
  (in-pattern
    (int 42)
    (if-guard
      (true))
    (send nil :puts
      (str "hello"))) nil)
```

See below for this guard semantics:
whitequark/parser#574 (comment)

changelog/new_support_guard_if_unless_of_pattern_matching_for_layout_space_around_keyword.md

@@ -0,0 +1 @@
+* [#9841](https://github.com/rubocop/rubocop/pull/9841): Support guard `if` and `unless` syntax keywords of Ruby 2.7's pattern matching for `Layout/SpaceAroundKeyword`. ([@koic][])

lib/rubocop/cop/layout/space_around_keyword.rb

@@ -69,6 +69,10 @@ def on_if(node)
           check(node, %i[keyword else begin end].freeze, 'then')
         end
 
+        def on_if_guard(node)
+          check(node, [:keyword].freeze)
+        end
+
         def on_in_pattern(node)
           check(node, [:keyword].freeze)
         end
@@ -117,6 +121,10 @@ def on_zsuper(node)
           check(node, [:keyword].freeze)
         end
 
+        def on_unless_guard(node)
+          check(node, [:keyword].freeze)
+        end
+
         def on_until(node)
           check(node, %i[begin end keyword].freeze)
         end

spec/rubocop/cop/layout/space_around_keyword_spec.rb

@@ -84,6 +84,15 @@
     it_behaves_like 'missing before', 'else', 'case a; in b; ""else end',
                     'case a; in b; "" else end'
     it_behaves_like 'missing after', 'else', 'case a; in b; else"" end', 'case a; in b; else "" end'
+
+    it_behaves_like 'missing before', 'if', 'case a; in "pattern"if "condition"; else "" end',
+                    'case a; in "pattern" if "condition"; else "" end'
+    it_behaves_like 'missing after', 'if', 'case a; in "pattern" if"condition"; else "" end',
+                    'case a; in "pattern" if "condition"; else "" end'
+    it_behaves_like 'missing before', 'unless', 'case a; in "pattern"unless "condition"; else "" end',
+                    'case a; in "pattern" unless "condition"; else "" end'
+    it_behaves_like 'missing after', 'unless', 'case a; in "pattern" unless"condition"; else "" end',
+                    'case a; in "pattern" unless "condition"; else "" end'
   end
 
   it_behaves_like 'missing before', 'elsif', 'if a; ""elsif b; end', 'if a; "" elsif b; end'