feat(config,render): allow customizing the debug icon (#1863)

## Description

lazy.nvim allows users to configure all icons except for the debug icon.
This PR enables user to configure the debug icon with `ui.icons.debug`

## Screenshots

Before:


![image](https://github.com/user-attachments/assets/42b02fd9-58e6-4ebc-a1a7-c5e91f07a11a)

After (with config `{ ui = { icons = { debug = ' ' } } }`):


![image](https://github.com/user-attachments/assets/3ade5392-a988-4a10-86fc-f52b41a690c5)

lua/lazy/core/config.lua

@@ -96,6 +96,7 @@ M.defaults = {
     icons = {
       cmd = " ",
       config = "",
+      debug = "●",
       event = " ",
       favorite = " ",
       ft = " ",