Allow presets in manifest

[dherman]

Allow target family presets in package.json (under neon.targets):

• neon rust-target recognizes presets in the manifest and knows how to expand them based on the FAMILY.json database
• neon add-target no longer automatically expands the preset in the manifest (if it doesn’t have to), allowing for more readable manifests
• integration test sniff-bytes uses presets in the manifest for testing

Example:

  "neon": {
    "type": "source",
    "org": "@sniff-bytes",
    "targets": [
      "linux",
      "macos"
    ]
  }