feat(dial): add checkbox augend for markdown (#5411)

## Description

Added support for markdown checkboxes. ( **[ ]** <-> **[x]** )

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  why we should accept this pull request. -->

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## Screenshots

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## Checklist

- [x] I've read the
[CONTRIBUTING](https://github.com/LazyVim/LazyVim/blob/main/CONTRIBUTING.md)
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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 = {