DEV: Added modifier to change mentions extracted from cooked text (#21654)

Added a new modifier hook to allow plugins to modify the @mentions
extracted from a cooked text.

Use case: Some plugins may change how the mentions are cooked to prevent
them from being confused with user or group mentions and display the user
card.

This modifier hook allows the plugin to filter the mentions detected or add new ways
to add mentions into cooked text.

lib/pretty_text.rb

@@ -457,6 +457,9 @@ module PrettyText
           end
         end
 
+    mentions =
+      DiscoursePluginRegistry.apply_modifier(:pretty_text_extract_mentions, mentions, cooked)
+
     mentions.compact!
     mentions.uniq!
     mentions

spec/lib/pretty_text_spec.rb

@@ -566,6 +566,45 @@ RSpec.describe PrettyText do
         ).to match_html '<p>Hello <span class="mention">@狮子</span></p>'
       end
     end
+
+    context "with pretty_text_extract_mentions modifier" do
+      it "allows changing the mentions extracted" do
+        cooked_html = <<~HTML
+        <p>
+          <a class="mention" href="/u/test">@test</a>,
+          <a class="mention-group" href="/g/test-group">@test-group</a>,
+          <a class="custom-mention" href="/custom-mention">@test-custom</a>,
+          this is a test
+        </p>
+        HTML
+
+        extracted_mentions = PrettyText.extract_mentions(Nokogiri::HTML5.fragment(cooked_html))
+        expect(extracted_mentions).to include("test", "test-group")
+        expect(extracted_mentions).not_to include("test-custom")
+
+        Plugin::Instance
+          .new
+          .register_modifier(:pretty_text_extract_mentions) do |mentions, cooked_text|
+            custom_mentions =
+              cooked_text
+                .css(".custom-mention")
+                .map do |e|
+                  if (name = e.inner_text)
+                    name = name[1..-1]
+                    name = User.normalize_username(name)
+                    name
+                  end
+                end
+
+            mentions + custom_mentions
+          end
+
+        extracted_mentions = PrettyText.extract_mentions(Nokogiri::HTML5.fragment(cooked_html))
+        expect(extracted_mentions).to include("test", "test-group", "test-custom")
+      ensure
+        DiscoursePluginRegistry.clear_modifiers!
+      end
+    end
   end
 
   describe "code fences" do