Changes:
- Rename InitExpression -> ConstantExpression in places which reference
  the ConstantExpression type.
- Move ConstantExpression to its own file, along with ValueOrError and
  EvaluateConstantExpression.

Change-Id: Ife572d783531216b6ea3d2626e4fbf4048463253
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3702798Reviewed-by: Clemens Backes <clemensb@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81204}