Add mechanism to exclude files/folders

[psolymos]

Sometimes we have directories that we want to exclude (i.e. scripts that we use to test ideas). These can be part of a .gitignore file, but sometimes these are not listed. We need to be able to tell what to exclude. Maybe by building on top of https://github.com/rstudio/renv/issues/880 and https://rstudio.github.io/renv/reference/dependencies.html#ignoring-files or something similar, like a .depsignore file. See https://github.com/rstudio/renv/blob/main/R/renvignore.R