Allow String + <other primitive type> concatenation within placeholders

[jdidion]

In previous WDL versions, concatenation of String to other primitive types was allowed, presumably to be used within placeholders. In development, those operations have been removed from the table. There is a new section on concatenation of optional strings during interpolation. I propose that section be expanded to allow String + <other primitive> concatenation within placeholders for both optional and non-optional operands.

[jdidion]

Addressed in v1.1