Adding datastore2json & json2datastore scripts

[Lucas-C]

This is a follow-up on a previous PR: #1823

I made sure that the script works with the latest version of Shaarli,
and added the "inverse" script, that convert from JSON to a datastore.php file.

Usage:

composer datastore2json > datastore.json
composer json2datastore datastore.json > data/datastore.php

Or:

php -f datastore2json > datastore.json
php -f json2datastore datastore.json > data/datastore.json

[Lucas-C]

To give some context, in my usage of Shaarli, those scripts serve as a base to perform several operations on all my bookmarks:

• remove a tag from all bookmarks
• replace a tag by one or several others in all bookmarks
• perform a regex-based substitution in the descriptions of all bookmarks

[SansGuidon]

This would be awesome.