Built-in functions for R are stale

[alexr00]

Describe the bug
Latest R contains more functions, e.g. this includes isTRUE() but not isFALSE().

isFALSE() was part of R 3.5.0, circa April 2018:

https://github.com/r-lib/backports#backports-for-r-versions-prior-to-350
https://github.com/wch/r-source/tree/tags/R-3-5-0

To Reproduce

isFALSE()
isTRUE()

Can you fix this issue by yourself? (We appreciate the help)

Yes / No
This issue was originally filed by @MichaelChirico, and they made a PR to the VS Code repo: microsoft/vscode#194352

Expected behavior
isFALSE and isTRUE should get the same scope from the R textmate grammar.