[ruby/pp] Only do RubyVM patches if class exists

This class does not exist in any implementation except CRuby.

I would recommend moving this code somewhere else, like a separate
file loaded only on CRuby or into CRuby itself. For now this
change is sufficient to load the library on other implementations.

https://github.com/ruby/pp/commit/7d5a220f64

lib/pp.rb

@@ -539,37 +539,39 @@ class MatchData # :nodoc:
   end
 end
 
-class RubyVM::AbstractSyntaxTree::Node
-  def pretty_print_children(q, names = [])
-    children.zip(names) do |c, n|
-      if n
-        q.breakable
-        q.text "#{n}:"
-      end
-      q.group(2) do
-        q.breakable
-        q.pp c
+if defined?(RubyVM::AbstractSyntaxTree)
+  class RubyVM::AbstractSyntaxTree::Node
+    def pretty_print_children(q, names = [])
+      children.zip(names) do |c, n|
+        if n
+          q.breakable
+          q.text "#{n}:"
+        end
+        q.group(2) do
+          q.breakable
+          q.pp c
+        end
       end
     end
-  end
 
-  def pretty_print(q)
-    q.group(1, "(#{type}@#{first_lineno}:#{first_column}-#{last_lineno}:#{last_column}", ")") {
-      case type
-      when :SCOPE
-        pretty_print_children(q, %w"tbl args body")
-      when :ARGS
-        pretty_print_children(q, %w[pre_num pre_init opt first_post post_num post_init rest kw kwrest block])
-      when :DEFN
-        pretty_print_children(q, %w[mid body])
-      when :ARYPTN
-        pretty_print_children(q, %w[const pre rest post])
-      when :HSHPTN
-        pretty_print_children(q, %w[const kw kwrest])
-      else
-        pretty_print_children(q)
-      end
-    }
+    def pretty_print(q)
+      q.group(1, "(#{type}@#{first_lineno}:#{first_column}-#{last_lineno}:#{last_column}", ")") {
+        case type
+        when :SCOPE
+          pretty_print_children(q, %w"tbl args body")
+        when :ARGS
+          pretty_print_children(q, %w[pre_num pre_init opt first_post post_num post_init rest kw kwrest block])
+        when :DEFN
+          pretty_print_children(q, %w[mid body])
+        when :ARYPTN
+          pretty_print_children(q, %w[const pre rest post])
+        when :HSHPTN
+          pretty_print_children(q, %w[const kw kwrest])
+        else
+          pretty_print_children(q)
+        end
+      }
+    end
   end
 end