esm: add support for JSON import assertion

Remove V8 flag for import assertions, enabling support for the syntax;
require the import assertion syntax for imports of JSON.

Support import assertions in user loaders.

Use both resolved module URL and import assertion type as the key for
caching modules.

Co-authored-by: Geoffrey Booth <webadmin@geoffreybooth.com>

PR-URL: https://github.com/nodejs/node/pull/40250


Reviewed-By: Bradley Farias <bradley.meck@gmail.com>
Reviewed-By: Michaël Zasso <targos@protonmail.com>
Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>