fix: task list rendering (Fix #749) (#757)

This PR restores the task list presentation removed in 4.8:

- Add “task-list-item” class to task list `<li>` elms
- Hide list bullets on unordered task lists `<li>` elms

It also provides several improvements on the pre-4.8 presentation:

- Add “task-list” class to task list `<ul>` elms 
- Display list numbers on ordered task lists `<li>` elms
- Render accessible task list items by wrapping checkbox and text in `<label>` elm
- Allow task lists to be nested within standard ordered/unordered lists 

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src/themes/basic/_layout.styl

@@ -383,6 +383,9 @@ body.sticky
   border-radius 2px
   padding 1rem
 
+.markdown-section ul.task-list > li
+  list-style-type none
+
 body.close
   .sidebar
     transform translateX(- $sidebar-width)