Add type or other extra field key for `isDigitalResource`

[wlpotter]

We will not have a separate CBSS relation key (cf. #36 ) for digital editions, e.g. something like the Digital Corpus. Instead, we will either tag these as item type of "Digital Resource" (if possible with the CSL item types), or we will add a boolean extra field key for this

[wlpotter]

We'll need to add the boolean flag as there aren't any options in CSL item types (cf. https://www.zotero.org/support/kb/item_types_and_fields#additional_item_types_and_fields)