Function is_executable_chunk 

pub fn is_executable_chunk(info_string: &str) -> bool

Return true if the chunk header denotes an executable Quarto chunk.

Executable engines are identifiers like r, python, julia, bash - the first character is ASCII alphabetic. Pandoc attribute fences such as {.python} (display block with a class) and raw-format fences like {=html} are not executable and must not be flagged by MD078/MD079.