types(DeepMergeAllFn): refine DeepMergeAllFn return type when handling lists with deep partial objects

[simonepizzamiglio]

Summary

This PR fixes an issue in the DeepMergeAllFn type where the returned type did not correctly match the type of the first object in scenarios where a list of objects was passed, and the subsequent objects were deep partial versions of the first.

Problem

Previously, when DeepMergeAllFn was called with a list of objects, where all objects from the second onward were deep partial versions of the first object, the return type defaulted to DeepMergeAll<{}, T> instead of preserving the type of the first object. This led to incorrect typings and made the function’s behavior less predictable.

The screenshot below shows the error before the changes in this PR: the deepmerge function would return a partial object type.

Checklist

• run npm run test and npm run benchmark
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows the Developer's Certification of Origin
  and the Code of conduct

[simonepizzamiglio]

The AllDeepPartialOrSame type does not handle cases where, starting from the second element onward, a non-deep partial version of the first item is passed. This happens because the condition Rest extends { [index: number]: DeepPartial<First> } would evaluate to true, even in such scenarios.

Closing because this is not feasible due to TS constraints.