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}