Properly parse Eshell variable splices for interactive completion

Properly parse Eshell variable splices for interactive completion

Previously, the code simply ignored the splice operator, which usually
worked, but isn't actually correct.

* lisp/eshell/em-cmpl.el (eshell-complete-eval-argument-form): New
function.
(eshell-complete-parse-arguments): Properly parse variable splices.

* test/lisp/eshell/em-cmpl-tests.el
(em-cmpl-test/parse-arguments/variable/splice): New test.