DEV: adds a addChatDrawerStateCallback API (#20640)

Usage:

```javascript
api.addChatDrawerStateCallback(({ isDrawerActive, isDrawerExpanded }) => {
  // do something
});
```

Note this commit also uses this new API to add a css class (chat-drawer-active) on the body when the drawer is active.

plugins/chat/spec/system/drawer_spec.rb

@@ -54,6 +54,20 @@ RSpec.describe "Drawer", type: :system, js: true do
     end
   end
 
+  context "when toggling open/close" do
+    it "toggles a css class on body" do
+      visit("/")
+
+      chat_page.open_from_header
+
+      expect(page.find("body.chat-drawer-active")).to be_visible
+
+      drawer.close
+
+      expect(page.find("body:not(.chat-drawer-active)")).to be_visible
+    end
+  end
+
   context "when going from drawer to full page" do
     fab!(:channel_1) { Fabricate(:chat_channel) }
     fab!(:channel_2) { Fabricate(:chat_channel) }