Add config option for whitelisting custom schemas

[flightofthefox]

Hello,

Thanks for this crate. Great to see such a feature-complete implementation!

I'm porting my code from my bespoke/spaghetti deno_core runtime over to this crate and have hit a snag WRT to loader permissions.

Using regular deno_core I was able to define extensions like the following:

extension!(
    sessions_ext,
    ops = [op_get_current_identity],
    esm_entry_point = "proven:sessions",
    esm = [ dir "src/extensions", "proven:sessions" = "sessions.js" ],
);

These extensions could then be used inside user-provided runtime code like:

import { getCurrentIdentity } from "proven:sessions";

However, in rustyscript the InnerLoader permissions checker will error with Error: Runtime("unrecognized schema for module import: proven:sessions") unless the module begins with ext:.

Switching the extension to use ext:sessions also results in deno_core [correctly] throwing Error: Runtime("Importing ext: modules is only allowed from ext: and node: modules.").

This pull request creates an additional runtime option which allows such custom schemas to be whitelisted. Allowing for a similar setup to deno_core. E.g.

let mut schema_whlist = HashSet::new();
schema_whlist.insert("proven:".to_string());
let mut runtime = Runtime::new(RuntimeOptions {
    schema_whlist,
    extensions: vec![sessions_ext::init_ops_and_esm()],
    ..Default::default()
})?;

Thanks again!

[rscarson]

Looks great!! Thanks!!
Wonder if it may be worth altering the extension example to showcase this?

[flightofthefox]

Looks great!! Thanks!! Wonder if it may be worth altering the extension example to showcase this?

No problem. Updated now :)