Add support for list item checkboxes (e.g. [X])

see https://orgmode.org/manual/Checkboxes.html
We're deviating from Org mode regarding the assigned css classes but the chosen
classes feel better than (on, off, trans) and it's not like the export matches
1:1 otherwise.

README.org

@@ -10,15 +10,7 @@ Take a look at [[https://niklasfasching.github.io/go-org/][github pages]] for so
     - see [[https://github.com/kaushalmodi/ox-hugo/blob/8472cf2d8667754c9da3728255634e8001a1da6d/ox-hugo.el#L1785-L1850][ox-hugo]] for auto generation
 - improve list parsing
   - handle list items with empty first line
-  - handle checkboxes & statistic cookies
   - handle ordered list overrides [@10]
-** TODO list checkboxes & statistics [0/2]
-https://orgmode.org/manual/Checkboxes.html
-1. [ ] parse checkbox of list item
-   - convert to html radio button unchecked, indeterminate (visual only), checked
-2. [ ] parse statistic cookies of headlines & first line of list items
-   -convert to =[sub 1<sup>1</sup>&frasl;<sub>2</sub>] || [90%]=
-
 * differences to goorgeous
 To get a feeling take a look at goorgeous vs go-org html rendering of the examples [[https://niklasfasching.github.io/go-org/go-org-vs-goorgeous][comparison]].
 Please note that a visual comparison is not fair to goorgeous as the stylesheet is not adapted to it.