Per https://tc39.es/proposal-import-assertions/#sec-assert-clause-to-assertions,
import assertions should be sorted by the import assertion [[Key]]s,
in order to prevent hosts from relying on a changing order of the
assertions to determine behavior.

Prior to this change, the assertions were being sorted by pointer. With
this CL, the keys are sorted using a code point ordering so that the
order of the assertions received by the host will be stable and
non-surprising.

This CL also switches the SourceTextModuleDescriptor's ModuleRequestMap,
RegularExportMap, and RegularImportMap to use the code point order
comparison rather than their former shortlex sort.  This change will not
be externally visible, but it seems best to make these consistent.

In order to avoid #including the fairly large ast-value-factory.h
into ast/modules.h, I changed ImportAssertions into a separate class
definition rather than keeping it as a typedef.  The alternative would
be to define a common AstRawStringComparer in ast-value-factory.h and
then #include ast-value-factory.h in both ast/modules.h and
parsing/parser.h so that the ImportAssertions typedef would have a
full, shared definition of the AstRawStringComparer type.

Bug: v8:10958
Change-Id: I29c9544aa0a4340c56e1ee631be6cabb2a2eb921
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2533038
Commit-Queue: Dan Clark <daniec@microsoft.com>
Reviewed-by: Marja Hölttä <marja@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71165}