Eshell variable expansion should always return strings inside quotes

Eshell variable expansion should always return strings inside quotes

This is closer in behavior to regular shells, and gives Eshell users
greater flexibility in how variables are expanded.

* lisp/eshell/esh-util.el (eshell-convert): Add TO-STRING argument.

* lisp/eshell/esh-var.el (eshell-parse-variable-ref): Add MODIFIER-P
argument and adjust how 'eshell-convert' and 'eshell-apply-indices'
are called.
(eshell-get-variable, eshell-apply-indices): Add QUOTED argument.

* test/lisp/eshell/esh-var-tests.el (eshell-test-value): New defvar.
(esh-var-test/interp-convert-var-number)
(esh-var-test/interp-convert-var-split-indices)
(esh-var-test/interp-convert-quoted-var-number)
(esh-var-test/interp-convert-quoted-var-split-indices)
(esh-var-test/interp-convert-cmd-string-newline)
(esh-var-test/interp-convert-cmd-multiline)
(esh-var-test/interp-convert-cmd-number)
(esh-var-test/interp-convert-cmd-split-indices)
(esh-var-test/quoted-interp-convert-var-number)
(esh-var-test/quoted-interp-convert-var-split-indices)
(esh-var-test/quoted-interp-convert-quoted-var-number)
(esh-var-test/quoted-interp-convert-quoted-var-split-indices)
(esh-var-test/quoted-interp-convert-cmd-string-newline)
(esh-var-test/quoted-interp-convert-cmd-multiline)
(esh-var-test/quoted-interp-convert-cmd-number)
(esh-var-test/quoted-interp-convert-cmd-split-indices): New tests.

* doc/misc/eshell.texi (Arguments): Expand this section, and document
the new behavior.
(Dollars Expansion): Provide more detail about '$(lisp)' and
'${command}' forms.

* etc/NEWS (Eshell): Announce this change (bug#55236).