Summary

Added the new rule useUnifiedTypeSignature, which disallows overload signatures that can be unified into a single signature.

Overload signatures that can be merged into a single signature are redundant and should be avoided. This rule helps simplify function signatures by combining overloads by making parameters optional and/or using type unions.

Example (Invalid): Overload signatures that can be unified:

function f(a: number): void;
function f(a: string): void;

interface I {
    a(): void;
    a(x: number): void;
}

Example (Valid): Unified signatures:

function f(a: number | string): void {}

interface I {
    a(x?: number): void;
}

Example (Valid): Different return types cannot be merged:

interface I {
    f(): void;
    f(x: number): number;
}

Test Plan

Tests are included

Notes

1. I had to include an implementation of type AST comparison in order to implement this rule. Let me know if it is already implemented anywhere else. Or (best case) if we can generate that. Basically need an AST comparison regardless of CST stuff.
2. As per discussion at 📎 Implement nursery/useUnifiedTypeSignature - typescript-eslint/unified-signatures #50, only adjacent overload signatures are checked.

Closes #50