repository: Add defineModelClass helper and fix DeepPartial<AnyObject>

[bajtos]

See #4235 for background context.

Implement a new helper function that allows LB4 users to create
model classes dynamically at runtime. This is similar functionality
as offered by LB3, but with the benefit of compile-time type checks.

Example use:

const definition = new ModelDefinition('Product')
  .addProperty('id', {type: 'number', id: true})
  .addProperty('name', {
    type: 'string',
  });

const Product = defineModelClass<
  typeof Entity,
  {id: number; name: string}
>(
  Entity,
  definition,
);

While testing this code, I found a bug in DeepPartial<T> that I have experienced before. This time I could not ignore it because one of my tests would fail, so I am finally fixing it (see the first commit). Consider the following code:

function create<T extends object>(data?: DeepPartial<T>) {
  // ...
}

Before this change, create<AnyObject>({key: 'value'}) is rejected by the compiler, becase "No overload matches this call."

With this change in place, the compiler no longer complains.

Checklist

👉 Read and sign the CLA (Contributor License Agreement) 👈

• npm test passes on your machine
• New tests added or existing tests modified to cover all changes
• Code conforms with the style guide
• API Documentation in code was updated
• Documentation in /docs/site was updated
• Affected artifact templates in packages/cli were updated
• Affected example projects in examples/* were updated

👉 Check out how to submit a PR 👈

[emonddr]

Very cool