parallel append: suppress generation of "dta_source" variable

Hello! Is there a way to suppress the generation of "dta_source" during a run of parallel append? This would be a useful feature when parallel is being used under memory constraints: "dta_source", because it is a file path and file name, is an arbitrarily long string. It can easily double the file size.

Thanks for all your work on the package!

Start by locating the parallel append implementation and the code that creates the dta_source variable. Determine how generation can be suppressed without changing the append result or parallel execution, then verify that a run with the option leaves dta_source absent and avoids storing the file path.

Written by the indexing model from the issue text.