Warning needed if regex contains DFDL Entities

Users will write dfdl:lengthPattern="(.+?)(?=%CR;%LF;--)"

Then wonder why it doesn't work (witness DFDL-792).

Turns out, DFDL entities aren't allowed in regular expressions, so the user has to write:

dfdl:lengthPattern="(.+?)(?=\r\n--)"

We should examine properties that are supposed to be regex, and warn if they contain things that look like DFDL entities.