[DFDL-WG] dfdl:extraEscapedCharacters - should be able to be expression

A user has noticed that for DFDL escape schemes based on escape characters, the properties dfdl:escapeCharacter and dfdl:escapeEscapeCharacter can both be runtime-valued expressions, but not dfdl:extraEscapedCharacters. This is not consistent. A major motivation for having these properties be runtime expressions is EDI formats, and the example motivating the need for dfdl:extraEscapedCharacters to be an expression also comes from EDIFACT. See this Daffodil ticket with a link to the email thread discussing this: https://issues.apache.org/jira/browse/DAFFODIL-2876 Mike Beckerle Apache Daffodil PMC | daffodil.apache.org OGF DFDL Workgroup Co-Chair | www.ogf.org/ogf/doku.php/standards/dfdl/dfdl Owl Cyber Defense | www.owlcyberdefense.com