Add a new 'DONE' keyword to todo-comments

I want 'DONE' to have the same fire emoji/icon and coloring as the
'HACK' comment, so that I can mark TODO comments as DONE in my TODO
files more easily.

Author: TheDDMbassy

lua/plugins/todo-comments.lua

@@ -6,9 +6,13 @@ return {
     'nvim-lua/plenary.nvim',
   },
   opts = {
-    signs = false,
+    signs = true,
     highlight = {
       comments_only = false,
     },
+    merge_keywords = true,
+    keywords = {
+      HACK = { alt = { 'DONE' } },
+    },
   },
 }