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

Fixes #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.

modules/markup/markdown/renderconfig.go

@@ -20,6 +20,9 @@ type RenderConfig struct {
 	TOC      string // "false": hide,  "side"/empty: in sidebar,  "main"/"true": in main view
 	Lang     string
 	yamlNode *yaml.Node
+
+	// Used internally.  Cannot be controlled by frontmatter.
+	metaLength int
 }
 
 func renderMetaModeFromString(s string) markup.RenderMetaMode {