markup/goldmark: Add Position to CodeblockContext

But note that this is not particulary fast and the recommendad usage is error logging only.

Updates #9574

markup/goldmark/codeblocks/transform.go

@@ -40,6 +40,7 @@ func (*Transformer) Transform(doc *ast.Document, reader text.Reader, pctx parser
 		}
 
 		codeBlocks = append(codeBlocks, cb)
+
 		return ast.WalkContinue, nil
 	})