Config: Don't validate the options under the engine key

[marcoroth]

This pull request updates @herb-tools/config and herb-config to accept the engine section as an opaque mapping, so that engine options shipped by the gem don't make herb-lint and herb-format reject the configuration file.

Both the JavaScript and the Rust config implementations mirrored the engine options and validated them strictly, even though neither of them ever reads anything under engine.

That section is only consumed by Herb::Engine on the Ruby side, where Herb.configuration.engine_option(key, default) is a free-form lookup that doesn't validate keys at all. The result was that any engine option the gem gained had to be added to two more schemas, in two more languages, and released in lockstep before it could be used.

Resolves #1994

[github-actions]

🌿 Interactive Playground and Documentation Preview

A preview deployment has been built for this pull request. Try out the changes live in the interactive playground:

• Playground Preview
• Documentation Preview
• Website Preview

---

_{🌱 Grown from commit cbf12a2}

---

✅ Preview deployment has been cleaned up.

[pkg-pr-new]

npx https://pkg.pr.new/@herb-tools/formatter@2000

npx https://pkg.pr.new/@herb-tools/language-server@2000

npx https://pkg.pr.new/@herb-tools/linter@2000

commit: cbf12a2