Prevent publishing packages that use import assertions

[nicolo-ribaudo]

This is related to denoland/deno#17944.

The old import assertions proposal (import ... assert { ... }) has been replaced by import attributes (import ... with { ... }). There are three platforms that currently support the old syntax: Deno, Node.js and Chrome. They all also support the new syntax (which is also supported by Bun and Safari).

As far as I know, Deno is the only platform that doesn't have a concrete plan yet to remove assert:

• Node.js will drop it in v22 (next week)
• Chrome will drop it in v126 (in June)

A good step to prevent more users from depending on the old syntax would be to ban it on JSR, so that new packages are forced to use the new syntax.