[transition] Combine ExpectedTransition{Key|Target} into ExpectedTransition

ExpectedTransitionKey directly returns key if TransitionAccessor has
only one member, and returns null if TransitionAccessor has multiple
entries.
This CL compare string content if TransitionAccessor has less than 8
entries.
This CL can improve the below micro case by ~17%.
  for (var i = 0; i < 1e7; ++i)
    JSON.parse('{"title":0,"completed":1,"id":2}');

Change-Id: I11307c5e68940a65ade244714652df6f8d655fbb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/5672333


Commit-Queue: Hao A Xu <hao.a.xu@intel.com>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#95142}