Enhance/fix `isatty` using `command test`.
  Presently, `isatty` only works on a handful of keywords. Here it
  is rewritten to be able to take any path, device or fd number as
  an argument, and eliminates errors printed to stdout.
  Per discussion in #1228, using `builtin test -c` within a pipe to
  test special file descriptors is not viable, so this implementation
  specifcially uses `command test`. Additionally, a note has been
  added to the documentation of `test` regarding this potential
  aberration from the expected output of the test utility under the
  'Standards' section.