Modify payload formatters according to the LoRaWAN® Codec API specification

[matthias-bs]

See https://resources.lora-alliance.org/technical-specifications/ts013-1-0-0-payload-codec-api

[X] Check existing scripts for standard compliance
[X] Create metadata.json file
[X] Create examples
[ ] Merge uplink_formatter.js and downlink_formatter.js into index.js
[ ] Create npm package

[matthias-bs]

Not sure if and when I will complete this.