feat: first-class Deno installs

[guybedford]

This adds support for first-class Deno installs via the recently-added Deno provider.

Specifically:

• generator.install('deno:path') corresponds to a stdlib install and will populate a versioned stdlib mapping for path to the Deno standard librariy.
• generator.install('denoland:oak') corresponds to a third-party Deno.land install and will populate the mapping similarly.

In addition a new defaultRegistry option is added as well as the ability for the providers object to provide registry mappings.

defaultRegistry applies the default registry for generator.install('x') to default to generator.install('registry:x') with the default obviously being npm:.

providers having custom registry: entries allows custom registry provider mappings via new Generator({ providers: { 'npm:': 'unpkg', 'denoland:': 'custom' } }) sort of thing ensuring this all is consistent with the provider and registry separation model.

[fusionstrings]

Love this, many thanks 🥇