Optionally clone Hermes into the App's directory

[kraenhansen]

I'm not certain that this is what we actually want, so I'm putting this up as a draft, just in the interest of transparency and starting a conversation.
No need to review this in detail, just yet.

I write this as part of playing around with using the packages outside of the mono-repo.

Merging this PR will:

• Clone hermes into the app's package root instead the host package's root.

If we merge this we'd have to instruct the user to add hermes to their gitignore.

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[kraenhansen]

Made up my mind already 🙈 There's no real need for this.