Add the ability to ignore schemes

[zephraph]

There are situations in which a platform will introduce an import scheme that aren't supported by deno. These schemes should still be publishable to JSR.

The biggest one right now that I'm aware of is cloudflare's cloudflare:* scheme. For example, if you're creating a durable object:

import { DurableObject } from "cloudflare:workers"

Likewise with importing env

import { env } from "cloudflare:workers"

Right now packages that reference these imports can't be published to JSR even though JSR ostensibly supports cloudflare as a suggested deployment target.

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See also #986

[crowlKats]

We are more inclined in adding specific schemes currently, so we will add support for cloudflare: specifiers