Move more of Eshell range handling to the parser phase

Move more of Eshell range handling to the parser phase

* lisp/eshell/esh-util.el (eshell-range): New struct.
(eshell--range-string-p, eshell--string-to-range): New functions.

* lisp/eshell/esh-arg.el (eshell-parse-integer)
(eshell-parse-range-token): New functions...
(eshell-parse-argument-hook): ... add them.
(eshell--after-range-token-regexp): New defsubst.
(eshell-concat-1): Don't remove the 'number' property; we use that when
handling range arguments.
(eshell--range-token): New constant.
(eshell-unmark-range-token): New function.

* lisp/eshell/esh-var.el (eshell-parse-index): Update implementation to
use parsed range argument.

* test/lisp/eshell/esh-var-tests.el (esh-var-test/interp-var-indices):
Test range index using variables.

(cherry picked from commit ed9ea57e57a915e743100591d7a71d44a4b4c0e9)