Support xeus-r kernel

[ryanlovett]

I'm converting an old LaTeX-based book covering the fundamentals of R and it'd be wonderful if myst could run live R-based kernels. xeus-r appears to be actively developed by QuantStack and it supports the code I'd like to run in the textbook. There is also jupyterlite-webr-kernel which I'm not as familiar with.

(related)

I've tried configuring myst to use xeus-r but the kernel connection fails very early on. I've since learned that this is expected at this point in development.

Cc'ing @fperez as requested.