Refactor register, create, and config loading so they can be composed manually by consumers

[cspotcode]

Partially implemented by #1474

Motivation: to support ts-node-esm, plus improve the API in general.

Should be able to read configuration without creating a Service nor loading any extra dependencies.

While we're at it, we should allow create()ing a service and then passing it to register(). Should not be forced to create and register at once.

import * as tsNode from 'ts-node';
// Not to be confused with current, internal readConfig, which is narrower in scope
const config = tsNode.readConfig(/*ReadConfigOptions, similar to CreateOptions*/);
const service = createFromConfig(config);
register(service); // Can we do this with overload? Do we need new `registerService()` function?

Will allow ts-node-esm to read config, determine it needs to spawn child process, spawn child, and then for child to skip the config parsing step and call create and register.