feat(dial): add checkbox augend for markdown

lua/lazyvim/plugins/extras/editor/dial.lua

@@ -140,6 +140,11 @@ return {
           }),
         },
         markdown = {
+          augend.constant.new({
+            elements = { "[ ]", "[x]" },
+            word = false,
+            cyclic = true,
+          }),
           augend.misc.alias.markdown_header,
         },
         json = {