Add ability to pack objects rather than just files

[xyzzy42]

This allows compilePack() src argument to not just be a directory with files to pack, but an Iterable of objects to pack.

Then it can be used to create packs that are dynamically created.

PF2e Workbench can use this for a compendium of macros it creates, where the macros don't exist as directory of json files. The macro Items are created dynamically. Currently, it uses an old NeDB format compendium for these and makes the database "manually" by concatenating lines of json text.