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.

Breaking changes

• The checkManifest function now returns ExtendableSnapFiles, which contains an ExtendableSnapManifest.

---

Note

Enables manifest inheritance by allowing a manifest to extends another, producing a merged manifest for tooling.

• Core: Add loadManifest to recursively load/merge manifests (cycle detection), introduce ExtendableManifest/ExtendableSnapFiles and DeepPartial; update schema to allow extends.
• Validation: Validators read from manifest.mergedManifest and write fixes to manifest.mainManifest; checkManifest now returns ExtendableSnapFiles (BREAKING).
• Webpack/Watch: SnapsWatchPlugin takes manifestPath and watches all files from loadManifest; config and snapshots updated.
• Dev server: /snap.manifest.json serves the merged manifest JSON; allowed static paths computed from merged manifest.
• Examples: bip32/snap.manifest.dev.json now extends base manifest.
• Tests/Coverage: Extensive new/updated tests; coverage numbers adjusted.

^{Written by Cursor Bugbot for commit 4eabac7. This will update automatically on new commits. Configure here.}

[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.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.34%. Comparing base (15a9666) to head (4eabac7).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3802   +/-   ##
=======================================
  Coverage   98.33%   98.34%           
=======================================
  Files         422      422           
  Lines       12073    12127   +54     
  Branches     1875     1885   +10     
=======================================
+ Hits        11872    11926   +54     
  Misses        201      201           

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[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

[FrederikBolding]

Do we care that these objects are plain? E.g. versus isObject?

Same question for a lot of this file.

[Mrtenz]

Not really, but unsure when to use isObject or isPlainObject or what the benefit is of one over the other 😅

[Mrtenz]

Updated to isObject.