fix(lib): reorder Array#reduce/reduceRight overloads in lib.d.ts

[taronsung]

Summary

Reorder the overload declarations for reduce and reduceRight in both ReadonlyArray<T> and Array<T> interfaces so the generic <U> overload (with initialValue: U) is checked before the same-type overload (with initialValue: T).

Problem

When calling reduce with an initial value of a different type than the array elements, TypeScript incorrectly infers the return type. This is because TypeScript checks overloads in declaration order, and the initialValue: T overload matches before the generic <U> overload gets a chance.

Example from #7014:

type UnaryFunction = (arg1: any) => any;
type BinaryFunction = (arg1: any, arg2: any) => any;

let binaryFuncs: BinaryFunction[] = [];
let unaryFunc: UnaryFunction = arg1 => {};
let reduced = binaryFuncs.reduce((prev, next) => prev, unaryFunc);

// BEFORE: Error - Type 'BinaryFunction' is not assignable to type 'UnaryFunction'
// AFTER: Works correctly - reduced is inferred as UnaryFunction
let f: UnaryFunction = reduced;

Solution

Swap the order of the two overloads that take an initialValue parameter:

• Before: reduce(cb, initialValue: T) then reduce<U>(cb, initialValue: U)
• After: reduce<U>(cb, initialValue: U) then reduce(cb, initialValue: T)

The same change is applied to both reduce and reduceRight in both ReadonlyArray<T> and Array<T>.

Fixes #7014

[taronsung]

@microsoft-github-policy-service agree

[RyanCavanaugh]

@typescript-bot test it

[typescript-bot]

Hey @RyanCavanaugh, this PR is in an unmergable state, so is missing a merge commit to run against; please resolve conflicts and try again.