fix(mcpack): replicate external subplatform support from mcmanifest

[HipsterBrown]

External subplatform (a.k.a custom device targets) support was added in Moddable v8.1.0 but only in mcmanifest.js for mcconfig to resolve. This PR creates parity support for mcpack as well.

[phoddie]

Thanks for catching this and for making the change to keep them in sync.

I noticed recently that xs-dev defaults to mcpack for new projects. That's a great way to make sure mcpack gets exercised.

[HipsterBrown]

Yea, my approach for xs-dev these days is to be a bridge for JS developers to the embedded world by providing a familiar workflow, I.e. package.json as the primary manifest, the polyfilled APIs provided by mcpack, and ECMA-419 first.

It will always support manifest.json and all the ways Moddable projects that can be built.