Make RuboCop::AST::IfNode support then?

This PR makes `RuboCop::AST::IfNode` support `then?`,
similar to `RuboCop::AST::InPatternNode` and `RuboCop::AST::WhenNode`.

changelog/new_make_rubocop_ast_if_node_support_then_predicate.md

@@ -0,0 +1 @@
+* [#341](https://github.com/rubocop/rubocop-ast/pull/341): Make `RuboCop::AST::IfNode` support `then?`. ([@koic][])

lib/rubocop/ast/node/if_node.rb

@@ -25,6 +25,13 @@ def unless?
         keyword == 'unless'
       end
 
+      # Checks whether the `if` node has an `then` clause.
+      #
+      # @return [Boolean] whether the node has an `then` clause
+      def then?
+        loc_is?(:begin, 'then')
+      end
+
       # Checks whether the `if` is an `elsif`. Parser handles these by nesting
       # `if` nodes in the `else` branch.
       #

spec/rubocop/ast/if_node_spec.rb

@@ -124,6 +124,32 @@
     end
   end
 
+  describe '#then?' do
+    context 'with `then` keyword' do
+      let(:source) do
+        <<~SOURCE
+          if foo? then
+            1
+          end
+        SOURCE
+      end
+
+      it { is_expected.to be_then }
+    end
+
+    context 'without `then` keyword' do
+      let(:source) do
+        <<~SOURCE
+          if foo?
+            1
+          end
+        SOURCE
+      end
+
+      it { is_expected.not_to be_then }
+    end
+  end
+
   describe '#elsif?' do
     context 'with an elsif statement' do
       let(:source) do