feat: return detected framework version accuracy

[serhalp]

Summary

@netlify/build-info currently uses two mechanisms for detecting a detected framework's version: a high-fidelity check of the installed package in node_modules and a low-fidelity fallback check based on the version range in package.json dependencies. It does this automatically and opaquely.

Much like the existing framework accuracy, this PR adds a framework version accuracy field on the result, for improved transparency.

Each detection result now has:

package: {
  name?: string
  version: string | 'unknown'
  versionAccuracy?: VersionAccuracy // new!
}

enum VersionAccuracy {
  // High accuracy: read from installed package in node_modules
  NodeModules = 'node_modules',
  // Medium accuracy: exact pinned version from package.json (e.g., "1.2.3")
  PackageJSONPinned = 'package_json_pinned',
  // Low accuracy: parsed from package.json dependency range (e.g., "^1.2.3")
  PackageJSON = 'package_json',
}

[serhalp]

I couldn't help myself here. This was unnecessarily complicated. Instead of just merging two record objects, we were turning them into an array [[k1, v1], [k2, v2], ...] for some reason, which is less readable, more complicated, and presumably less performant (we use .find() on this, twice).