Add support for chaining conditionals in Eshell

Add support for chaining conditionals in Eshell

* lisp/eshell/esh-cmd.el (eshell-structure-basic-command): Check for the
presence of the conditional.  Allow any number of BODY forms.
(eshell-rewrite-if-command): Add support for 'else' keyword and chained
conditionals.

* test/lisp/eshell/esh-cmd-tests.el (esh-cmd-test/if-else-statement):
Test 'else' keyword.
(esh-cmd-test/if-else-statement-chain): New test.

* doc/misc/eshell.texi (Control Flow): Document this change.

* etc/NEWS: Announce this change.

(cherry picked from commit fada04cfc788a486265c9da6636611986b48ae49)