Fix parsing of indices in Eshell expansions

Fix parsing of indices in Eshell expansions

Previously, more-complex index expansions, like '$var[":" 0]' or
'$var[$(expr) 0]' failed to parse correctly.

* lisp/eshell/esh-var.el (Commentary): Clarify indexing and length
expansions.
(eshell-parse-indices): Expand docstring and support parsing inside
double-quotes.
(eshell-eval-indices): New function.
(eshell-parse-variable): Use it.

* test/lisp/eshell/esh-var-tests.el (eshell-test-value): New defvar.
(esh-var-test/interp-var-indices,
(esh-var-test/interp-var-split-indices)
(esh-var-test/interp-var-string-split-indices)
(esh-var-test/interp-var-regexp-split-indices)
(esh-var-test/interp-var-assoc, esh-var-test/interp-var-length-list)
(esh-var-test/interp-var-length-string)
(esh-var-test/interp-var-length-alist)
(esh-var-test/quoted-interp-var-indices)
(esh-var-test/quoted-interp-var-split-indices)
(esh-var-test/quoted-interp-var-string-split-indices)
(esh-var-test/quoted-interp-var-regexp-split-indices)
(esh-var-test/quoted-interp-var-assoc)
(esh-var-test/quoted-interp-var-length-list)
(esh-var-test/quoted-interp-var-length-string)
(esh-var-test/quoted-interp-var-length-alist): New tests.

* doc/misc/eshell.texi (Dollars Expansion): Expand and reword
documentation for indexing and length expansions.