`getChanges` should be renamed to `$getChanges` in mongoose 10

[AbdelrahmanHafez]

Prerequisites

• I have written a descriptive issue title
• I have searched existing issues to ensure the issue has not already been raised

Issue

Basically the title. We should also go over the rest of the internal properties/methods and make sure they all start with $, if anyone uses getChanges as a method or a property name they'll now be unable to use getChanges.

Maybe we can even introduce an alias $getChanges now, and for the rest of the properties, if any, and make the original getChanges emit a deprecation warning.

[vkarpov15]

I thought about this some more and I don't agree. For one thing, getChanges() is part of public API, it isn't purely an internal property. I don't think we should rename all Mongoose specified properties to start with $. For example, we won't rename doc.deleteOne() -> doc.$deleteOne(). We could consider adding a $getChanges alias to be consistent with methods like model and save - we have $model() and $save() for cases when people have document properties named model or save.

[AbdelrahmanHafez]

You're right. Adding aliases is the right move.
Should/do we emit a warning when someone uses a public API property in a schema?

[vkarpov15]

Yes, for example Mongoose emits a warning "save" is a reserved schema pathname warning if you have a path named save, and you can pass a suppressReservedKeysWarning schema option to suppress that warning.

[AbdelrahmanHafez]

Cool, so what's remaining here is to:

1. Add similar warning to all reserved schema pathnames
2. Verify all public API methods have $ aliases