build: migrate Fastly config script to the official fastly-js client

[cwillisf]

Proposed Changes

• Replace the unmaintained fastly@2.2.0 (thisandagain) client with the official fastly@15 (fastly-js).
• Rewrite bin/lib/fastly-extended.js to wrap the official per-resource API classes (Version/Condition/Header/ResponseObject/Purge), keeping the same promise-returning, upsert-by-name helper signatures.
• Drop the util.promisify shims in bin/configure-fastly.js; call the now promise-native helpers directly.
• Modernize both files to const/let, arrow functions, and template literals.

Reason for Changes

• fastly@2.2.0 is unmaintained and callback-based. The official fastly-js client is promise-native, maintained, and covers the full API (including VCL snippets, which the follow-up redirect-config rework will use).
• Removes the deprecated request-based dependency subtree.

Test Coverage

• Behavior-preserving: the same conditions, headers, and response objects are created.
• Verified locally: node --check, eslint --no-ignore clean, and a smoke test (module loads, authenticates, and a missing-service error propagates through the script's .catch).
• Not yet exercised against a live Fastly service: the create/update API calls, the 404-upsert fallback, and param shapes should be confirmed on a staging deploy.
• One behavior change: authenticate() fails fast at startup if FASTLY_API_KEY is unset, rather than deferring to the first API call.

[Copilot]

Pull request overview

This PR migrates the Fastly configuration tooling from the legacy callback-based fastly@2.2.0 client to the official promise-native fastly@15.1.0 client, updating the wrapper helpers and the configuration script accordingly.

Changes:

• Upgrade dependency from fastly@2.2.0 to fastly@15.1.0 (and update lockfile).
• Rewrite bin/lib/fastly-extended.js to use fastly-js resource API classes with promise-based, upsert-by-name helpers.
• Simplify bin/configure-fastly.js by removing util.promisify usage and calling promise-native helpers directly.

Reviewed changes

Copilot reviewed 1 out of 4 changed files in this pull request and generated 1 comment.

File	Description
package.json	Updates fastly dependency version to the official client.
package-lock.json	Reflects new dependency tree after migrating to fastly@15.1.0.
bin/lib/fastly-extended.js	Replaces the legacy wrapper with promise-based helpers built on fastly-js APIs.
bin/configure-fastly.js	Removes promisify shims and updates callers to use the new promise-based wrapper.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[cwillisf]

As of right now, this is the latest version of fastly, and overriding superagent to a non-deprecated version by force would mean jumping several major versions. Best case, that's unsupported territory; worst case, it just plain doesn't work. I'm sure fastly will upgrade their superagent dep at some point.