Raw in the sense that these chunks don't contain pure R code, but they
contain a header and footer of markdown. Only code chunks that have an engine
whose name matches engine-pattern are considered as R code.
identify_raw_chunks(lines, filetype, engine_pattern = get_engine_pattern())A character vector of lines from an Rmd or Rnw file.
A string indicating the filetype - either 'Rmd' or 'Rnw'.
A regular expression that must match the engine name.