Extend rules for Extensions dictionaries to model PDF 2.0 ISO/TS requirements

[petervwyatt]

There are now many ISO TS extensions to PDF 2.0 that have explicit requirements regarding the Document Catalog Extensions entries that must be present, but these rules are not currently encoded in the Arlington data model.

Extend Extensions.tsv to specifically include an "ISO_" prefix and then create data model files for each official ISO TS extension with their required key/value data as per the TS. This doesn't preclude other developer extensions using other prefixes since they will match * in Extensions.tsv.

Here is the list of currently published ISO/TS documents but more are in the pipeline.