Update headings list to display plain text, and add option to insert at beginning

src/core/todo-transformer.ts

@@ -49,6 +49,16 @@ export function removeTodoLine(content: string, lineNumber: number): string {
 	return lines.join('\n');
 }
 
+/**
+ * Insert a todo line at the beginning of the content.
+ */
+export function insertTodoAtBeginning(content: string, todoLine: string): string {
+	if (!content) {
+		return todoLine;
+	}
+	return todoLine + '\n' + content;
+}
+
 /**
  * Insert a todo line at the end of the content.
  * Handles files with/without trailing newlines.