Use callr instead of processx to launch shinyapp

[cderv]

Here https://github.com/ColinFay/crrry/blob/247f44edee49de2c74cb61862a2d0d18a201a446/R/launch.R#L14-L23

callr is build on top on top processx and have feature that are useful to control R from R

Could be too much but could also be useful on the run to help maintain, because callr has been thought as using processx for controling R.