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/renderconfig.go

@@ -18,6 +18,9 @@ type RenderConfig struct {
 	TOC      bool
 	Lang     string
 	yamlNode *yaml.Node
+
+	// Used internally.  Cannot be controlled by frontmatter.
+	metaLength int
 }
 
 // UnmarshalYAML implement yaml.v3 UnmarshalYAML