feat(Validation): Add unique validator constraint

[EmanFateen]

Overview

In this PR, I have introduced a new validation constraint called unique.

export type UniqueOptions = ConstraintOptions<{
  message?: string;
  normalizer?: (value: unknown) => unknown;
  fields?: string[];
}>;

It's exposed through the builtInConstraints and can be used like

const rules = {
  tags: [{ unique: {} }],
  emails: [{ unique: { normalizer: value => typeof value === 'string' ? value.trim() : value } }],
  coordinates: [{ unique: { fields: ['latitude', 'longitude'] } }],
  usernames: [{ unique: { groups: ['create'] } }],
};

• Main purpose of this constraint is to validate whether the given collection contains only unique elements.
• By default, uniqueness is checked with strict comparison semantics.
• The constraint supports a normalizer to transform values before uniqueness is checked.
• The constraint also supports fields to validate uniqueness based on a selected combination of object fields.
• This constraint preserves the current optional-field behavior, which means if the value is undefined the constraint will return no violations.

Changes made:

1. Added the unique constraint under comparison constraints.
2. Covered the required cases in unique.test file.
3. Exposed the unique constraint through the builtInConstraints object.

Q	A
License	GPLv3
Issue	Closes #170 >