There's a bit more work to do to add support for import assertions for
dynamic import().  This is the first of a series of changes to do that.

This adds parser support for the form of import() that takes import
assertions per https://tc39.es/proposal-import-assertions/#prod-ImportCall

A future change will pass the assertions expression along to
Runtime_DynamicImportCall where the assertions will be unpacked and
filtered per Isolate::supported_import_assertions_.

Bug: v8:10958
Change-Id: Ib1c80d15ac44923d97c5fdfcc4bd732cb9245cf9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2612038Reviewed-by: Adam Klein <adamk@chromium.org>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#71960}