Fix for Flexible Sync Configuration

[takameyer]

What, How & Why?

This makes the flexible boolean incompatible with paritionValue,
without setting tsconfig to false. It does however not allow,
flexible to be false, but that's a better tradeoff in the end.

This closes #4552

☑️ ToDos

• 📝 Changelog entry
• 📝 Compatibility label is updated or copied from previous entry
• 🚦 Tests
• 📱 Check the React Native/other sample apps work if necessary
• 📝 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

[takameyer]

Just for a little more background on these changes, I found that instead of trying to make a type that supports {flexible: false, partitionValue: "something"}, we could just make {flexible: false} completely incompatible and only support {flexible: true, partitionValue: never} and {flexible: never, partitionValue: "something"}.
This has the advantage of fixing the type errors we were having with strict: false.
I think this is a good tradeoff, as I do not really see the use case for flexible: false.

[tomduncalf]

👍 Thanks for finding this workaround! I've only made minor suggestions which you can choose to ignore if preferred