Allow using dollar expansions in Eshell conditionals

Allow using dollar expansions in Eshell conditionals

* lisp/eshell/esh-cmd.el (eshell-structure-basic-command): Forms
beginning with 'eshell-escape-arg' are "data-wise".

* test/lisp/eshell/esh-cmd-tests.el (esh-cmd-test/while-loop)
(esh-cmd-test/until-loop, esh-cmd-test/if-statement)
(esh-cmd-test/if-else-statement, esh-cmd-test/unless-statement)
(esh-cmd-test/unless-else-statement): Use variable interpolation.
(esh-cmd-test/while-loop-ext-cmd, esh-cmd-test/until-loop-ext-cmd)
(esh-cmd-test/if-else-statement-ext-cmd)
(esh-cmd-test/unless-else-statement-ext-cmd): New tests, adapted from
the existing ones.

* doc/misc/eshell.texi (Control Flow): Update documentation for
conditionals (bug#57129).