Add() becomes AddLocalExport, which takes an export_name and a local_name.
New parsing tests exercise this.
Also start generating exports for default exports (though this doesn't yet
handle anonymous default exports).
BUG=v8:1569
LOG=n
Review URL: https://codereview.chromium.org/934323004
Cr-Commit-Position: refs/heads/master@{#26758}
