add new validation rule for 'money' formats that can be negative

Create a new validation rule similar to CRM/Utils/Rule.php::money() except that it would allow both positive and negative money values.

The current money function uses preg_match against
'/(^{\d+\.\d?\d?$)|(}\.\d\d?$)/'

A new function (eg. 'moneyposneg' or 'moneysigned') could use preg_match
'/(^{?\d+\.\d?\d?$)|(}?\.\d\d?$)/'

With this new validation rule, it would allow a selection of custom field type to be 'money - positive' or 'money - signed', each with their corresponding validation rule.

There is not a use case for this in any of the built-in money fields in CiviCRM, but I ran into this limitation when I wanted to add a custom field for a rebate amount.