This also adds a new VariableMode, IMPORT, which will be
used to do appropriate binding for Import-declared Variables.
Only named imports are handled for now. "import *" and default
import syntaxes have had their TODOs adjusted to match the new
code structure.
BUG=v8:1569
LOG=n
Review URL: https://codereview.chromium.org/948303004
Cr-Commit-Position: refs/heads/master@{#26895}
