[validation] Union related ESLint rules

[flobernd]

Introduction

Most statically typed languages do not provide first class support for generic union types.

Besides missing language support, unions are often hard to disambiguate during deserialization.

ESLint rules to implement

1. Prefer tagged variants

Condition: Use of union A | B | ... for a field/property when all members are class types.
Severity: 🔴 error
Message: Use tagged variants instead (external or internal).
Exceptions: Exceptions only allowed for legacy APIs. Never suppress for new APIs.

Use internal tagged variants to model a form of polymorphism that is as well broadly supported by JSON serializer libraries.

Use external tagged variants (containers), if the individual union variants are not directly related.

2. Do not use inline unions

Condition: Use of union A | B | ... for a field/property.
Severity: 🔴 error
Message: Use an alias type instead.
Exceptions: None.

Defining an alias type export type MyUnion = A | B | ... and using this as the field/property type instead of the inline union, allows to hand-craft the type in some statically typed languages which greatly improves usability or even is the only way to correctly some union types at all.

cc @pquentin @l-trotta @Anaethelion