Add Object.linkingObjectCount()

[astigsen]

What, How & Why?

This adds Object.linkingObjectCount() method to get the total number of incoming links to an object.

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 🚦 Tests
• 📝 Public documentation PR created or is not necessary
• 💥 Breaking label has been applied or is not necessary

If this PR adds or changes public API's:

• typescript definitions file is updated
• jsdoc files updated
• Chrome debug API is updated if API is available on React Native

[nirinchev]

In .NET this is called BacklinksCount: https://github.com/realm/realm-dotnet/blob/master/Realm/Realm/RealmObject.cs#L108. Not sure what's the name in other SDKs but we should probably align :)

[astigsen]

I also thought of naming it backlinksCount(), but in the end I found it more correct to align the naming with the other method on the object which is called linkingObjects() (in .NET this looks to be named GetBacklinks(), so there it would match up).

[kneth]

👍 expect minor comment.

[kneth]

The parameter should be documented too. They can be copied from the method above.

[fealebenpae]

Looking at the C++ implementation this method shouldn't take any parameters.

[astigsen]

Yes, it should not take any parameters. It was a copy/paste mistake. Fixed.

[kneth]

One - unrelated - test is failing in one build. I have seen it before, and I haven't a clear understanding of why it fails. I believe we can ignore it for now (see also #1748).