Fix task list checkbox toggle to work with YAML front matter (#25184) (#25236)

Backport https://github.com/go-gitea/gitea/pull/25184 by @jtran
Closes #25225.

Fixes https://github.com/go-gitea/gitea/issues/25160.

`data-source-position` of checkboxes in a task list was incorrect
whenever there was YAML front matter. This would result in issue content
or PR descriptions getting corrupted with random `x` or space characters
when a user checked or unchecked a task.

(cherry picked from commit 1650a26eb5)

modules/markup/markdown/ast.go

@@ -76,7 +76,8 @@ func IsSummary(node ast.Node) bool {
 // TaskCheckBoxListItem is a block that represents a list item of a markdown block with a checkbox
 type TaskCheckBoxListItem struct {
 	*ast.ListItem
-	IsChecked bool
+	IsChecked      bool
+	SourcePosition int
 }
 
 // KindTaskCheckBoxListItem is the NodeKind for TaskCheckBoxListItem
@@ -86,6 +87,7 @@ var KindTaskCheckBoxListItem = ast.NewNodeKind("TaskCheckBoxListItem")
 func (n *TaskCheckBoxListItem) Dump(source []byte, level int) {
 	m := map[string]string{}
 	m["IsChecked"] = strconv.FormatBool(n.IsChecked)
+	m["SourcePosition"] = strconv.FormatInt(int64(n.SourcePosition), 10)
 	ast.DumpHelper(n, source, level, m, nil)
 }