fix(lint/useConst): handle reads before assignment and improve docs

[Conaclos]

Summary

Fix #2575

I also took the opportunity of fixing an issue I discovered when reading the code:
The rule should not report an implicitly initialized variable that is read before its assignment.
The following example was previously reported:

let a;
a;
a = 0;

Test Plan

I added 3 tests and updated the snapshots.

[codspeed-hq]

CodSpeed Performance Report

Merging #2584 will not alter performance

_{Comparing conaclos/useConst/improve-rule (77a6815) with main (15ca346)}

Summary

✅ 93 untouched benchmarks