`extract_r_source()` is overly eager with detecting knitr patterns

[AshesITR]

It seems the test code is linted as Rmd for some reason?!
The NA should be investigated in a separate issue. Maybe we stumbled on an actual bug in the extractor code.

Originally posted by @AshesITR in #1403 (comment)

[AshesITR]

The reason is knitr:::detect_pattern() falls back to returning the first pattern set which matches any chunk begin or inline code if the extension is unknown (which is the case for extension .R).

It's probably safe to assume that any file which parses as R code should be linted as R code, even if it happens to match a knitr pattern.