Add value_omission method to AST::PairNode

Follow up to whitequark/parser#818

This PR adds `value_omission` method to `AST::PairNode` for Ruby 3.1's
hash value omission.
It can replace string match with that named API.

changelog/new_add_value_omission_to_pair_node.md

@@ -0,0 +1 @@
+* [#10219](https://github.com/rubocop/rubocop/pull/10219): Add `value_omission` method to `AST::PairNode` for Ruby 3.1's hash value omission. ([@koic][])

lib/rubocop/ast/node/pair_node.rb

@@ -62,6 +62,13 @@ def inverse_delimiter(*deprecated, with_spacing: deprecated.first)
       def value_on_new_line?
         key.loc.line != value.loc.line
       end
+
+      # Checks whether the `pair` uses hash value omission.
+      #
+      # @return [Boolean] whether this `pair` uses hash value omission
+      def value_omission?
+        source.end_with?(':')
+      end
     end
   end
 end

spec/rubocop/ast/pair_node_spec.rb

@@ -711,4 +711,18 @@
       end
     end
   end
+
+  describe '#value_omission?' do
+    context 'when using hash value omission', :ruby31 do
+      let(:source) { '{ x: }' }
+
+      it { expect(pair_node).to be_value_omission }
+    end
+
+    context 'when not using hash value omission' do
+      let(:source) { '{ x: x }' }
+
+      it { expect(pair_node).not_to be_value_omission }
+    end
+  end
 end