DMN15 — Change to the "at literal" in FEEL

Currently the at literal = "@", string literal
Rather than a string following the @ symbol we would use the ISO 8601 format.
For date : @YYYY-MM-DD e.g. @2019-09-17
For time : @THH:MM:SS e.g. @T09:55:00
For date and time : @YYYY-MM-DDTHH:MM:SS e.g. @2019-09-17T09:55:00
For duration : @P[n]Y[n]M[n]DT[n]H[n]M[n]S where [n] is a number
e.g. @P18M, @P365D, PT48H