Perform a best-effort check for module context and provide an
appropriate error.

As seen from the import-blah-script.js test, we could have invalid
import expressions in a script context that could result in an error
saying "Cannot use import statement outside a module" which isn't
the ideal error because the error is an incorrect import
expression.

But, when the developer changes to a module context, the
correct error is thrown.

To fix this, we'd have to refactor and call ParseImportDeclaration,
and then throw an appropriate error, which seems like a lot of
overhead for not enough gain.

Bug: v8:9392, v8:6513
Change-Id: I520ebb490fff4d95743a7c751d4095db9a35d41b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1675948Reviewed-by: Mythri Alle <mythria@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sathya Gunasekaran <gsathya@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62358}