Add support for markovchain package

[takahser]

Currently it's not possible to install markovchain using the R web terminal:

> install.packages("markovchain")
Warning message:
In install.packages("markovchain") :
  Requested package markovchain not found in webR binary repo.

I also tried providing the cran, but that didn't work either:

> install.packages(c("markovchain"), repos = "http://cran.us.r-project.org")

Warning: unable to access index for repository http://cran.us.r-project.org/bin/emscripten/contrib/4.3:
  download from 'http://cran.us.r-project.org/bin/emscripten/contrib/4.3/PACKAGES' failed
Warning message:
In install.packages(c("markovchain"), repos = "http://cran.us.r-project.org") :
  Requested package markovchain not found in webR binary repo.

On my local environment, both operations succeed (MacBook Pro M2).