Add unused group name mutation

- Mutates capture group names: `/(?<foo>bar)/` -> `/(?<_foo>bar)/`.
- Closes #573. NOTE: I have deviated from the issue slightly and chosen a `_` prefix so that, like with local variables, names can be left in unused if they are explicitly marked as such. I have decided to extend the behavior to not mutating to a non-capture group as well in this case because the `_` prefix seems like a clear choice that the user would rather keep the label even if the capturing functionality is unused.

Changelog.md

@@ -1,3 +1,8 @@
+# Unreleased
+
+* [#1200](https://github.com/mbj/mutant/pull/1200)
+  * Add unused group name mutation:  `/(?<foo>bar)/` -> `/(?<_foo>bar)/`.
+
 # v0.10.25 2021-01-02-03
 
 * [#1194](https://github.com/mbj/mutant/pull/1194)

lib/mutant/mutator/node/regexp/named_group.rb

@@ -15,8 +15,22 @@ class NamedGroup < Node
           def dispatch
             return unless group
 
-            emit(s(:regexp_passive_group, group))
             emit_group_mutations
+
+            # Allows unused captures to be kept and named if they are explicitly prefixed with an
+            # underscore, like we allow with unused local variables.
+            return unless name_underscored?
+
+            emit(s(:regexp_passive_group, group))
+            emit_name_underscore_mutation
+          end
+
+          def emit_name_underscore_mutation
+            emit_type("_#{name}", group)
+          end
+
+          def name_underscored?
+            name.start_with?('_')
           end
         end # EndOfLineAnchor
       end # Regexp

meta/regexp/regexp_named_group.rb

@@ -15,4 +15,14 @@
 
   mutation '/(?:\w)/'
   mutation '/(?<foo>\W)/'
+  mutation '/(?<_foo>\w)/'
+end
+
+Mutant::Meta::Example.add :regexp_named_group do
+  source '/(?<_foo>\w)/'
+
+  singleton_mutations
+  regexp_mutations
+
+  mutation '/(?<_foo>\W)/'
 end