Add Extendr, the modern alternative to RCpp

[andy-thomason]

Extendr is an alternative to RCpp that uses Rust instead of C++

Authoring Rust packages and R functions is easier than with RCpp and benefits from a huge collection of packages available on crates.io

Many existing R packages use Extendr, which has was founded in 2020.

Extendr has a companion CRAN package rextendr which allows single functions to be compiled in R.