add exception for `library_call_linter()` when `library()` is invoked within a function body

[nicholas-masel]

add an exception for library() calls inside functions -- e.g. for libraries calling library() on behalf of the user. The current logic seems tailored to scripts, but the linter will end up being used in packages too.

[MichaelChirico]

Example: {callr} calling R on a subprocess using an anonymous function. Within the function, library() is called first, but not within the script calling it:

https://github.com/tidyverse/dplyr/blob/73742713bc61d4ef079e72b806143b23cb7c4871/inst/bench/bench-mean.R#L10-L13