asciidoctor/extract-header-ids: make work with definition lists

These were failing because the plugin was copied from an outdated documentation
and definition lists don't have the .context method:
https://github.com/asciidoctor/asciidoctor/issues/3363

README.adoc

@@ -16445,6 +16445,8 @@ explicitly-given
 
 One application we have in mind for this is that as of 2.0.10 Asciidoctor does not warn on header ID collisions between auto-generated IDs: https://github.com/asciidoctor/asciidoctor/issues/3147 But this script doesn't solve that yet as it would require generating the section IDs without the `-N` suffix. Section generation happens at `Section.generate_id` in Asciidoctor code.
 
+Hastily hacked from: https://asciidoctor.org/docs/user-manual/#https://asciidoctor.org/docs/user-manual/#tree-processor-example until I noticed that that example had a bug at the time and so fixed it here: https://github.com/asciidoctor/asciidoctor/issues/3363
+
 [[asciidoctor-link-target-up-rb]]
 === asciidoctor/link-target-up.rb
 

asciidoctor/extract-header-ids

@@ -8,17 +8,12 @@ require 'asciidoctor/extensions'
 class Main < Asciidoctor::Extensions::TreeProcessor
   def process document
     return unless document.blocks?
-    process_blocks document
+    (document.find_by context: :section).each do |section|
-    nil
+      if section.id
-  end
+        puts section.id
-
-  def process_blocks node
-    node.blocks.each_with_index do |block, i|
-      if block.context == :section
-        puts block.id
       end
-      process_blocks block if block.blocks?
     end
+    nil
   end
 end