Change $subscribe to a different name to not conflict with rx implementations

[ScottAwesome]

This is really an ergonomics idea.

If you use any of the major observables implementation, like rxjs they all use subscribe and unsubscribe to mean something with observables.

As an ergonomic standpoint, having both $subscribe and subscribe (and in some cases, if you follow the RX nomeclature, observables themselves may be named observed$ so you end up with something like observed$.subscribe), things become confusing to quickly grasp what you are or are not subscribing to in a code base.

The though here being so we don't have these naming clashes, is to rename $subscribe to $observe there by removing the nomenclature confusion.

I realize this is likely a small lift, however it could be done via deprecation and just aliasing $subscribe to $observe

Alternative naming of course, is welcome!

My hope is this makes it just a little more ergonomic to use rxjs and others with pinia