feat(value-objects) - Add value objects to mikro-orm

[mlusca]

In this pull request, we add Value Objects to our project. Value Objects are a software design practice that allows us to encapsulate values ​​with specific behaviors and business rules in immutable objects.

The addition of these Value Objects brings several advantages to our project:

1.Encapsulation: Each Value Object encapsulates validation logic and behavior related to the value it represents. This makes our code more modular and easier to maintain.
2. Immutability: Value Objects are immutable, which means that once created, their values ​​cannot be changed.
This helps prevent bugs and makes our code more predictable.
3. Re-usability: Value Objects can be reused in different parts of our code, which helps reduce code duplication.

Utilization

@Entity()
class User {

  @PrimaryKey()
  id!: number;

  @Property()
  email: Email; // Value Object
}

Example of Value-Object

import { ValueObject } from './value-object';

const REGEX = /^[a-z0-9.]+@[a-z0-9]+\.[a-z]+(\.[a-z]+)?$/i;

export class Email extends ValueObject<string, Email> {

  protected validate(value: string): boolean {
    return REGEX.test(value);
  }

}

[codecov]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (83aa6c8) 99.66% compared to head (ee70afb) 99.64%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
packages/core/src/value-objects/value-object.ts	89.18%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5000      +/-   ##
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- Coverage   99.66%   99.64%   -0.03%     
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  Files         221      225       +4     
  Lines       16204    16291      +87     
  Branches     3890     3909      +19     
==========================================
+ Hits        16150    16233      +83     
- Misses         54       56       +2     
- Partials        0        2       +2     

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[B4nan]

How is this different from embeddables (or custom types if it's about a single column, you can use those to validate the values too)? I don't think we need anything else for value objects. If something, we could improve the support for those two existing features, I don't want to add something "almost the same", it would only make things more confusing.

[mlusca]

**B4nan ** commented Dec 9, 2023 •

@B4nan
For me the point would be:

1. Simplicity. Just defining the type, without needing to customize the decorator, reducing the cognitive load.
2. Centralized business rule. Embedables are not the same thing and in my opinion custom types were not made for that.

At the moment I thought about these two points, but I believe there could be more.
If you don't like it, feel free to close this PR.