Parse the AssertEntries in an import assertion clause, storing them in
a map.  Plumb them through the parser to the appropriate
SourceTextModuleDescriptor methods.

The next change will plumb them into the SourceTextModuleDescriptor's
ModuleRequestMap and through to SourceTextModuleInfo::New.

Bug: v8:10958
Change-Id: I19c31090520f14f94d014e760f5fe372bf773fc2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2482326Reviewed-by: Marja Hölttä <marja@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#70622}