Refactor to work with v9 and fix various bugs

[vexofp]

Various changes to make the module compatible with v9 and to fix various bugs.

• Various data structure accesses updated to work with v9 and to handle edge cases more gracefully.
• The update hook was changed to a pre-update hook, so it should only fire on the updating client now. This should stop permissions errors from popping up on other clients.
• The manual sync mechanism was re-worked. Fixed the "finished" notification to actually wait for the sync to finish.
• Stopped the addition of the links section on completely empty journal entries, to avoid that content being incorrectly edited and saved by TinyMCE.

The core of how data is stored in the flags is unchanged, so I do not think there should be any data migration issues. If there are, a manual sync should resolve them since it wipes everything.

The module.json updates are something of a guess, as I am not terribly familiar with that part.

[AzfarBadaroudine]

This version works great for drag&drop links, but not for direct links.

For example, a journal entry named test1: The first line won't create a link in test1, but the second one will work fine

@JournalEntry[test1] @JournalEntry[HWYWzYV9XMItUHuL]{test1}

[jtracey]

I went ahead and forked + published this module with these changes (plus a couple other improvements). https://foundryvtt.com/packages/journal-backlinks/ https://github.com/jtracey/journal-backlinks