feat(openapi-diff): export functions to be imported into other node apps

[seriouslag]

Motivation and Context

This PR exposes the operations of the openapi-diff cli as functions for other programs to import.

Type of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• New feature (non-breaking change which adds functionality)

Details

This PR changes the entry point of the openapi-diff to export the core functionality and changes the CLI to run from a CLI file.
CLI functionality remains the same, so it is not a breaking change.

This allows the following in JS programs:

import { Differ } from '@fresha/openapi-diff';
import {
  OpenAPIObject,
  OpenAPIReader,
} from "@fresha/openapi-model/build/3.0.3";

const reader = new OpenAPIReader();
const spec1= reader.parseFromFile(inputPath1);
const spec2= reader.parseFromFile(inputPath2);

const differ = new Differ(spec1, spec2);
...