This is the first change in the process of implementing import
assertions per https://tc39.es/proposal-import-assertions/.

This CR adds support for the empty form of the AssertClause.

Also added is a --harmony-import-assertions flag to enable/disable
import assertions.  For now, the feature is off by default.

The next change will enable the parser to handle a non-empty list
of AssertEntries.

Bug: v8:10958
Change-Id: I0832d89effc27225aa4430605a51690461daf7ad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2468623Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#70545}