Implements Stage 4 proposal from http://rwaldron.github.io/exponentiation-operator/,
without adding any knowledge of the feature to compiler backends.

BUG=v8:3915
LOG=Y
R=adamk@chromium.org, rossberg@chromium.org, littledan@chromium.org

Review URL: https://codereview.chromium.org/1678303002

Cr-Commit-Position: refs/heads/master@{#34890}