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.

---

Note

Introduces extendable Snap manifests and updates tooling to load, merge, validate, and watch them.

• Adds extends support with deep-merge via new loadManifest, returning mergedManifest and involved files
• Refactors validators to operate on mergedManifest and write fixes to the base manifest; adds types (ExtendableManifest, ExtendableSnapFiles)
• Updates Webpack: SnapsWatchPlugin now takes manifestPath and watches files from loadManifest
• Dev server now serves JSON from mergedManifest and computes allowed paths from it
• Example snap.manifest.dev.json now extends base manifest; tests/snapshots adjusted accordingly

^{Written by Cursor Bugbot for commit 1a67620. 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 (1a67620).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3802   +/-   ##
=======================================
  Coverage   98.33%   98.34%           
=======================================
  Files         422      422           
  Lines       12073    12125   +52     
  Branches     1875     1884    +9     
=======================================
+ Hits        11872    11924   +52     
  Misses        201      201           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[cursor]

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

[cursor]

Incorrect assignment causes unnecessary manifest self-merge

Medium Severity

When a manifest has no extends field, extendedManifest is incorrectly set to baseManifest instead of undefined. This causes runFixes to call mergeManifests with the manifest against itself (since extendedManifest?.result returns a truthy value). The mergeManifests function's early-return check if (!extendedManifest) then fails, triggering an unnecessary deepmerge operation and creating a new object instead of preserving the original reference.