This turns the member functions CollectKeysTo and CopyEnumKeysTo of
BaseNameDictionary into helper function in keys.cc; they are only used
there. Further, CollectKeysTo is renamed to CollectKeysFromDictionary.

EnumIndexComparator is moved from keys.cc to dictionary.h.

All moves are motivated by
https://chromium-review.googlesource.com/c/v8/v8/+/2489692 needing
these function in other places.

Bug: v8:7569
Change-Id: Ia8039e98fd00cef45dec376f3c401635b2321761
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2509597Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Frank Emrich <emrich@google.com>
Cr-Commit-Position: refs/heads/master@{#70912}