cleanup: add renv-related entries to .Rbuildignore

so developers can use (untracked) renv's at the repo level without as many git
and CHECK complaints.

Author: lcbrooks

.Rbuildignore

@@ -1,3 +1,5 @@
+^renv$
+^renv\.lock$
 ^.*\.Rproj$
 ^\.Rproj\.user$
 ^LICENSE\.md$