Get disk usage in Deno

I am looking for something similar to the `npm:diskusage` package. It's based on GYP but a FFI port might not be that hard to do for someone more experienced with Deno. Is there anything in Deno land that does not rely on forking a child process (`df` and the like)?

Start by checking whether Deno already provides disk-usage APIs or modules that avoid forking `df`; compare the `npm:diskusage` package and its GYP implementation with the feasibility of an FFI port. Done means identifying a viable Deno-land solution or clearly documenting the remaining implementation path.

Written by the indexing model from the issue text.