use is.function() to check if a fn is a fn

[richierocks]

• stop() doesn't allow sprintf()-style formatting.
• inherits(x, "function") doesn't currently work for rlang lambda functions (https://github.com/r-lib/rlang/issues/735)

[codecov[bot]]

Codecov Report

Merging #227 into master will not change coverage. The diff coverage is 100%.

@@           Coverage Diff           @@
##           master     #227   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files          47       47           
  Lines        2631     2631           
=======================================
  Hits         2423     2423           
  Misses        208      208

Impacted Files	Coverage Δ
R/check-fun-def.R	98.82% <100%> (ø)	:arrow_up:

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