Type Index <-> Type registrations relationship

[NoelDeMartin]

As discussed in #17, type indexes are not behaving as expected at the moment and they are cumbersome to work with.

Ideally, it should be possible to do something like this:

const typeIndex = await SolidTypeIndex.find('https://my-pod.com/settings/privateTypeIndex');

// Read registrations
typeIndex.registrations; // SolidTypeRegistration[]

// Create registrations
typeIndex.relatedRegistrations.create({
	forClass: 'https://schema.org/Recipe',
	instanceContainer: 'https://my-pod.com/recipes/',
});

// Remove registrations
typeIndex.registrations[0].delete();

// etc.

This is partly already supported, but it's not feature-complete yet. In the mean-time, it should still be possible to work with type indexes but it's not as intuitive and requires deeper knowledge of the library.