Lazily convert numeric strings to Lisp numbers in Eshell

Lazily convert numeric strings to Lisp numbers in Eshell

This should reduce the number of issues with Eshell converting strings
to numbers too aggressively and losing information (e.g. "001" -> 1)
while still allowing almost all of the beneficial uses, like summing a
list of numeric strings with '+'.

* lisp/eshell/esh-util.el (eshell--do-mark-numeric-string): New
function.
(eshell-convert-to-number): Make obsolete in favor of...
(eshell-mark-numeric-string): ... this.  Update callers.

* lisp/eshell/esh-arg.el (eshell--numberlike-p): New function...
(eshell-concat-1): ... use it.

* test/lisp/eshell/esh-util-tests.el:  Reimplement type conversion tests
to use 'eshell-convertible-to-number-p' instead.

* test/lisp/eshell/esh-var-tests.el
(esh-var-test/interp-var-splice-concat, esh-var-test/interp-concat-cmd)
(esh-var-test/interp-convert-var-split-indices)
(esh-var-test/interp-convert-quoted-var-split-indices)
(esh-var-test/interp-convert-cmd-multiline)
(esh-var-test/interp-convert-cmd-split-indices): Adjust tests to check
the new behavior.

* doc/misc/eshell.texi (Type Conversion): New section.
(Expansion): Clarify concatenation behavior.

(cherry picked from commit 43d5b7a04c4b1a8a7d57f25824df2e8720d2c567)