Simplify types, address bugs, and improve type safety

[J0m1ty]

Changes:

• Reduces complexity and number of utility types
• Adds an optional type parameter to DiscriminatedUnion for type checking
• Fixes an unknown bug

Closes #1 and closes #2

[Copilot]

Pull Request Overview

A refactor to simplify and consolidate TypeScript utility types for discriminated unions and merged unions, improving type safety and addressing a bug.

• Replaced multiple internal helpers with UnionKeys, BuildMember, and Expand
• Updated DiscriminatedUnion signature to include an optional Tags parameter
• Simplified MergedUnion implementation and removed legacy merge utilities

Comments suppressed due to low confidence (3)

src/index.ts:36

• The JSDoc template tags (@template TDiscriminator and @template TStates) no longer match the updated generic parameters (Discriminator, Variants, Tags). Please update or add @template annotations to reflect Discriminator, Variants, and the optional Tags parameter.

* @template TDiscriminator - The string literal type that will be used as the discriminator property name

src/index.ts:78

• [nitpick] The type parameters A and B in MergedUnion<A, B> are not very descriptive. Consider renaming them to something like Left, Right or TFirst, TSecond to clarify their roles.

export type MergedUnion<

src/index.ts:78

• This new MergedUnion logic introduces complex conditional and mapped types; consider adding dedicated type-level tests (e.g., via tsd) to verify all merge scenarios and edge cases.

export type MergedUnion<

[J0m1ty]

We should add another testing library to check actual types
https://github.com/tsdjs/tsd