Digiline support for interacting with smartshops

[Emojigit]

Digilines would help set up dynamic shops (e.g. restock with another type of items once out of stock, or cycle through items). @jingkaimori wrote some codes based on AiTechEye's version: https://github.com/jingkaimori/minetest-mod-smartshop

[jingkaimori]

If a shop is dynamic, price of the item may change after player opens ui of shop, but shop do not tell such change with current implementation. My fork solves such inconsistent by holding price info since player opens ui, and warn player if current price does not consist with held price. To hold price during player opens ui, relevant information should be stored in the scope of api/formspec.lua and passed to instance of shop class through argument of methods.

[jingkaimori]

Possible features about digiline. Are those features reasonable and suitable?

1. Query info of storage
   1. remaining items in inventory
   2. protect setting
2. Query info of shop
   1. remaining items in inventory
   2. offers
   3. purchase history
   4. protect setting
   5. admin shop setting
   6. freebie setting
   7. connections
      • storage position
      • stoarge digiline channel
3. Change metadata of shop
   1. offers
   2. protect setting
   3. admin shop setting
   4. freebie setting
4. connect storage to shop
   • assign by position
5. notify on events
   1. item sold out
   2. purchase complete

[Emojigit]

Duplicate of #35.