FIX: adds post_quote as placeholder (#29083)

The script `send_chat_message` when used with the `post_created_edited` trigger now accepts `{{post_quote}}` as placeholder for the value of `message`.

This is made possible by a new method in `utils`. Usage:

```ruby
  placeholders["foo"] = utils.build_quote(post)
```

plugins/chat/plugin.rb

@@ -463,6 +463,7 @@ after_initialize do
     field :sender, component: :user
 
     placeholder :channel_name
+    placeholder :post_quote, triggerable: :post_created_edited
 
     triggerables %i[recurring topic_tags_changed post_created_edited]
 
@@ -471,6 +472,10 @@ after_initialize do
       channel = Chat::Channel.find_by(id: fields.dig("chat_channel_id", "value"))
       placeholders = { channel_name: channel.title(sender) }.merge(context["placeholders"] || {})
 
+      if context["kind"] == "post_created_edited"
+        placeholders[:post_quote] = utils.build_quote(context["post"])
+      end
+
       creator =
         ::Chat::CreateMessage.call(
           chat_channel_id: channel.id,