feat!: Allow manifest to extend another manifest

[Mrtenz]

This is a follow up to #3793, which introduces custom manifests (e.g., one for production, one for development), allowing a custom manifest to extend another manifest. Right now, the entire production manifest would need to be copied to the development manifest. After this change it's possible to do the following:

// snap.manifest.dev.json
{
  "extends": "./snap.manifest.json",
  "initialConnections": {
    "http://example.com": {}
  }
}

With a base manifest like:

// snap.manifest.json
{
  "proposedName": "My Snap",
  "other": "properties"
}

After which the manifests would be merged like this:

{
  "proposedName": "My Snap",
  "other": "properties",
  "initialConnections": {
    "http://example.com": {}
  }
}

Right now this is mainly useful for local development, but may be used for bundling production Snaps in the future.

[FrederikBolding]

Do we not have this somewhere already?

[Mrtenz]

Just one that works on objects in snaps-cli's test utils. I couldn't find a general one.