Support applying indices to more Eshell dollar expansions

Support applying indices to more Eshell dollar expansions

For example, '${echo -e "hi\nbye"}[1]' should expand to "bye".

* lisp/eshell/esh-var.el (eshell-parse-variable-ref): Support applying
indices to '${}', '$()', and '$<>' forms.
(Bug#54227)

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