stabilise rhai

[garypen]

Add support for rhai's module system and propose stabilising rhai.

This deepens the integration between rhai and the router and makes it possible to modularise
functionality within rhai code.

This change makes it possible for a router to have multiple rhai scripts loaded within a router. A single "main" script and an arbitrary number of "imported scripts" which are imported using rhai's module resolution mechanism from a specified location.

This kind of thing then becomes possible:

rhai/router.yaml:
===
rhai:
  scripts: rhai

rhai/also.rhai:
===
fn also() {
    print("also");
}

rhai/helpful.rhai:
===
fn helpful() {
    print("helpful");
}

rhai/main.rhai:
===
import "helpful.rhai" as helpful;
import "also.rhai" as also;

// Call map_request with our service and pass in a string with the name
// of the function to callback
fn subgraph_service(service, subgraph) {
    helpful::helpful(); <= calling functions from imported modules
    also::also(); <= calling functions from imported modules
    print(`registering request callback for ${subgraph}`);
    const request_callback = Fn("process_request");
    service.map_request(request_callback);
}

The above would process the "main" script as part of the config processing, including importing the specified modules.

You can do a lot of interesting things with rhai's modules, including executing them directly on inclusion or dynamically loading them within the scope of a function (or a closure). For more details:

https://rhai.rs/book/ref/modules/import.html