Fix evaluation of asynchronous expansions in Eshell indices

Fix evaluation of asynchronous expansions in Eshell indices

Previously, this code passed the indices to a separate function, which
called 'eval' on them, but it should instead make an S-expr that
'eshell-do-eval' can evaluate (bug#60942).

* lisp/eshell/esh-var.el (eshell-eval-indices): Mark obsolete.
(eshell-prepare-indices): New function...
(eshell-parse-variable): ... use it.  Also, remove irrelevant comment.
(eshell-parse-variable-ref): Fix quoting in docstring.
(eshell-parse-indices): Fix typo in docstring.

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/interp-var-indices-subcommand)
(esh-var-test/quoted-interp-var-indices-subcommand): New tests.