Added concept of plugins to simplify and generate configuration

[jsoverson]

This PR adds the concept of "plugins" to apex.

Goal: To simplify configuration and reduce the amount someone needs to know or change before getting started.

Secondary goal: Support dynamic configurations.

Implementation:

Apex plugins are TypeScript files that export a single default function that has the signature (doc: apex.Document, config: Configuration) -> Configuration. They take in a parsed apex document with an existing configuration and return a new configuration.

For example, given an apex configuration that looks like this:

spec: sample.axdl
plugins:
  - https://deno.land/x/sample_plugin@v.0.0.0/mod.ts

And where sample_plugin@v.0.0.0/mod.ts looks like this:

import { Configuration } from "https://deno.land/x/apex_cli@v0.0.6/src/config.ts";
import * as apex from "https://deno.land/x/apex_core@v0.1.0/mod.ts";

export default function (doc: apex.ast.Document, config: Configuration): Configuration {
  config.generates ||= {};

  const interfaces = doc.definitions.filter(
    (def) => def.getKind() === apex.ast.Kind.InterfaceDefinition
  ) as apex.ast.InterfaceDefinition[];

  for (const iface of interfaces) {
    config.generates[`./src/${iface.name.value}.ts`] = {
      module: "./some/generator/module.ts",
    };
  }

  return config;
}

Running generate would result in configuration that looks like this before getting passed to the generators:

spec: sample.axdl
plugins:
  - ./sample-plugin.ts
generates:
  ./src/Test.ts: 
    module: ./some/generator/module.ts

Note: The new configuration doesn't need to be dynamic. Plugins are also suitable for turning blobs of configuration into a single plugin definition.

[pkedy]

Instead of duplicating the logic in process.ts and generate.ts, could the plugins be added / iterated in the existing code path?

[pkedy]

I'm using deno fmt src to format all the code instead of using prettier. It looks like the formatter you are using is making a change here. Could you pls run deno fmt src?

[jsoverson]

PR is ready for review @pkedy.