The change https://chromium-review.googlesource.com/c/v8/v8/+/2572173
implemented HostGetSupportedImportAssertions [1] in a fairly literal
sense, where the host supplies a list of supported import assertions
and V8 filters the import assertions in a ModuleRequest and exposes
only supported assertions via its API surface.

However, we've decided that the interop guarantees provided
by doing the filtering in V8 are probably not worth the added
complexity. Thus, this change removes the filtering. Going forward,
hosts will be expected to ignore unknown asserions received from V8.

This is mostly a revert of
https://chromium-review.googlesource.com/c/v8/v8/+/2572173, with
v8::Isolate::CreateParams::supported_import_assertions being kept
for now (since we first have to delete the Blink code that sets it),
and a new comment in v8.h instructing hosts to ignore unknown
assertions.

[1] https://tc39.es/proposal-import-assertions/#sec-hostgetsupportedimportassertions

Bug: v8:10958
Change-Id: I7e8e2a7fbfe2d5bf891805cff6c3160d0e6825cd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2643563Reviewed-by: Marja Hölttä <marja@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Mythri Alle <mythria@chromium.org>
Commit-Queue: Dan Clark <daniec@microsoft.com>
Cr-Commit-Position: refs/heads/master@{#72299}