FEATURE: Bundle discourse-footnote plugin into core (#23995)

Formerly https://github.com/discourse/discourse-footnote

plugins/footnote/plugin.rb

@@ -0,0 +1,46 @@
+# frozen_string_literal: true
+
+# name: footnote
+# about: Adds markdown.it footnote support to Discourse
+# version: 1.0
+# authors: Discourse Team
+# url: https://github.com/discourse/discourse/tree/main/plugins/footnote
+
+enabled_site_setting :enable_markdown_footnotes
+
+register_asset "vendor/javascripts/markdown-it-footnote.js", :vendored_pretty_text
+
+register_asset "stylesheets/footnotes.scss"
+
+register_svg_icon "ellipsis-h" if respond_to?(:register_svg_icon)
+
+on(:before_post_process_cooked) do |doc, post|
+  doc
+    .css("a.footnote-backref")
+    .each do |backref|
+      href = backref["href"] || ""
+      id = href[6..-1].to_i
+      backref["href"] = "#footnote-ref-#{post.id}-#{id}"
+    end
+
+  doc
+    .css("sup.footnote-ref a")
+    .each do |ref|
+      href = ref["href"] || ""
+      id = href[3..-1].to_i
+      ref["href"] = "#footnote-#{post.id}-#{id}"
+
+      id = ref["id"] || ""
+      id = id[5..-1].to_i
+      ref["id"] = "footnote-ref-#{post.id}-#{id}"
+    end
+
+  doc
+    .css("li.footnote-item")
+    .each do |li|
+      id = li["id"] || ""
+      id = id[2..-1].to_i
+
+      li["id"] = "footnote-#{post.id}-#{id}"
+    end
+end