🚀 Feature: change `validate` to return rich `Result` type

[michaelfaith]

Feature Request Checklist

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Overview

This is the umbrella issue tracking the rollout of the change to validate's return type, to include the rich Result type, instead of just error messages for each top-level property. We'll do it in two phases:

1. Introduce the new return type as opt-in via optional parameter (Non-Breaking)
2. Change return type to be the new type and remove the optional param (Breaking)

Additional Info

Question: is this overkill? Should we just collapse 2 and 3 into one band-aid rip after enough time has passed with the opt-in behavior? cc @JoshuaKGoldberg thoughts? Updated to just be one breaking change

[JoshuaKGoldberg]

Yeah I think two breaking changes is kind of a lot. Let's just rip the band-aid off once. If folks really need it, we can always add it back in.