FEATURE: adds support for headings in chat (#29552)

```
```

Will now be converted into their html versions: <h1>, <h2>, <h3>, ...

plugins/chat/app/models/chat/message.rb

@@ -220,6 +220,7 @@ module Chat
       blockquote
       emphasis
       replacements
+      heading
     ]
 
     def self.cook(message, opts = {})

plugins/chat/spec/lib/chat/chat_pretty_text_spec.rb

@@ -1,14 +0,0 @@
-# frozen_string_literal: true
-
-#
-RSpec.describe Chat::Message do
-  describe ".cook" do
-    it "renders kbd inline tag" do
-      cooked = Chat::Message.cook <<~MD
-    <kbd>Esc</kbd> is pressed
-    MD
-
-      expect(cooked).to include("<p><kbd>Esc</kbd> is pressed</p>")
-    end
-  end
-end

plugins/chat/spec/models/chat/message_spec.rb

@@ -69,10 +69,34 @@ describe Chat::Message do
       )
     end
 
-    it "does not support headings" do
-      cooked = described_class.cook("## heading 2")
+    it "supports kbd" do
+      cooked = described_class.cook <<~MD
+      <kbd>Esc</kbd> is pressed
+      MD
 
-      expect(cooked).to eq("<p>## heading 2</p>")
+      expect(cooked).to match_html <<~HTML
+      <p><kbd>Esc</kbd> is pressed</p>
+      HTML
+    end
+
+    it "supports headings" do
+      cooked = described_class.cook <<~MD
+      # h1
+      ## h2
+      ### h3
+      #### h4
+      ##### h5
+      ###### h6
+      MD
+
+      expect(cooked).to match_html <<~HTML
+      <h1><a name="h1-1" class="anchor" href="#h1-1"></a>h1</h1>
+      <h2><a name="h2-2" class="anchor" href="#h2-2"></a>h2</h2>
+      <h3><a name="h3-3" class="anchor" href="#h3-3"></a>h3</h3>
+      <h4><a name="h4-4" class="anchor" href="#h4-4"></a>h4</h4>
+      <h5><a name="h5-5" class="anchor" href="#h5-5"></a>h5</h5>
+      <h6><a name="h6-6" class="anchor" href="#h6-6"></a>h6</h6>
+      HTML
     end
 
     it "supports horizontal replacement" do