Daffodil as a research vehicle, has some extensions that are not part of the DFDL standard, yet are worth retaining.

In order to retain these and still be in compliance with the standard, they need to be controlled via properties that are not in the DFDL standard namespace, but some extension namespace particular to Daffodil.

This task is to implement this URI, and identify and move Daffodil extensions, enabling them only when some daffodil-specific property enables them.

Example: expressions in daffodil can do array assignments. This is a powerful capability that actually enables DFDL to contain quite rich transformations such as array transposing. This should be enabled explicitly, and otherwise prohibited. A URI such as https://opensource.ncsa.illinois.edu/dfdl/daffodil/1.0/ext, and a property in that namespace such as x-enable-array-valued-expressions="true"