Make 'eshell-number-regexp' into a regular defvar

Make 'eshell-number-regexp' into a regular defvar

This isn't a very useful thing to customize, since it needs to detect
numbers that can successfully be parsed by 'string-to-number'.
Changes to this variable would therefore likely requiring adjusting
'eshell-convert-to-number' as well.

* lisp/eshell/esh-util.el (eshell-number-regexp): Make into a defvar
and improve the regexp to support more numbers (including infinity and
NaN).

* test/lisp/eshell/esh-util-tests.el
(esh-util-test/eshell-convert-to-number/floating-point)
(esh-util-test/eshell-convert-to-number/floating-point-exponent)
(esh-util-test/eshell-convert-to-number/non-numeric)
(esh-util-test/eshell-convert-to-number/no-convert): New tests.