generate .Renviron files from WhiteboxTools options

[brownag]

I am planning some improvements of documentation for WhiteboxTools environment variables and had an idea.

There should be a way to generate/append .Renviron files with the WhiteboxTools settings,
The .Renviron file would contain static paths and options so that subsequent loads of that project or on that machine have the users generated setup. It would support setting these env vars for an R session:

• R_WHITEBOX_EXE_PATH

• R_WHITEBOX_WD

• R_WHITEBOX_VERBOSE

• R_WHITEBOX_COMPRESS_RASTERS

• R_WHITEBOX_MAX_PROCS

• wbt_create_Renviron() - set target vars via list, settings.json or by name and create a .Renviron file or write to stdout (does not change package options until that file is read)

• Use base::readRenviron() (optionally) in wbt_init(). Look in R and/or WBT working directories?