Rename id in collection to _id

[codeit-ninja]

I think it a bit wierd that we cannot use the id field in a collection, it must be a string and cannot be a number.

The explaination as to why is in my opinion a bit wierd.
https://github.com/wovalle/fireorm/blob/master/docgen/Core_Concepts.md

In firestore the id is saved as _id, why not implement it like that? Not be able to use an number as id is pretty inconvenient.

[wovalle]

Hi @redbullzuiper! This was an early design choice made when building this library. Unfortunately to fix it now it'll require refactoring how the Collections and Sub-collections are accessed to which I don't have the required the time now.

Happy to reviewing this if anybody wants to jump in though

[codeit-ninja]

It will break the code probaly for alot of people, as they are now using the id field in their code. Not sure what the best solution is here.

Interesting to hear what others think about this.