feat: add Model.{getInitialModel,withoutScope}, rename schema+(un)?scope

[ephys]

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Description Of Change

Another building block for #14280

Context:
Model.scope, Model.unscope and Model.schema return new classes.
This is a problem for the above PR because that PR needs to be able to check whether two models classes are the same.

This PR does a few things:

• Deduplicate models generated by these 3 methods. Which means that Model.schema('public') === Model.schema('public')
• Adds Model.withInitialScope: unscope() is a misnomer, it doesn't restore the model to its state before the call to scope(). Instead it returns a model with no scope at all (including the default scope). withInitialScope() does return a model with the default scope.
• Adds Model.getInitialModel which returns the model, like it was before any call to scope, unscope or schema
• Renames unscope to withoutScope, scope to withScope and schema to withSchema.
  This change was done to signal that these methods will not mutate the Model and, instead, return a new Model. These names follow conventions for similar methods that we can find in other libraries.
  The old names still exist but will log a deprecation notice. There are no breaking changes in this PR.

[WikiRik]

Looks good! I'm assuming the documentation of the renames will follow later?

[ephys]

In our upgrade guide? Yep :)

[github-actions]

🎉 This PR is included in version 7.0.0-alpha.11 🎉

The release is available on:

• npm package (@release-v7 dist-tag)
• GitHub release

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