Use `export default` for main exported classes

[tidoust]

With the switch to ECMAScript modules, main Reffy functions were exported as named exports but there was no default export, meaning Reffy could be imported through:

import * as reffy from 'reffy';
import { crawlSpecs } from 'reffy';

... but there was no way to write:

import reffy from 'reffy';

This update makes it possible to do either:

import reffy from 'reffy';
import { crawlSpecs } from 'reffy';

Now, the dark side of this feature requires lines some might consider to be unnatural... In particular, that requires writing similar things 3 times in index.js:

1. First, to import named exports from the different modules
2. Second, to create named exports
3. Third, to create a default Reffy export, a sort of static class that exposes the functions.

There's no way to reduce that (if there is a way, I don't know how :)). That makes me wonder whether this approach is good practice?

For example, there is a specific syntax in JavaScript for re-exporting values from other modules:

export { crawlSpecs } from "./src/lib/specs-crawler.js";

That is tempting! But that syntax does not import crawlSpecs and so cannot be used to also create a default export with crawlSpecs as a property.

Also, while they are written the same way, the values of the export and export default syntaxes cannot be combined because they are conceptually different beasts: the former is a list of names, the latter an object with properties.

The update does similar magic for the parse-webidl.js module. The post-processor module only exports the processor as a sort of static class and does not create named exports, because that's how the module is used in practice.