Resolves the root directory used for Parquet data from an explicit argument
or the DATA_DIR environment variable. In an interactive session, when
neither is available, the helper asks the user to choose or enter a
directory and offers to persist it in either project-level .Renviron or
user-level ~/.Renviron.
Usage
pq_data_dir(data_dir = NULL, prompt = interactive(), fallback = ".")