Add Homebridge Config UI for Script2 with on_off/stateless sections and schema updates

[pponce]

Motivation

• Provide a richer Homebridge Config UI X experience by exposing separate on_off_switches and stateless_switches sections and a custom UI rather than relying on raw JSON devices only.
• Preserve backward compatibility with existing devices legacy configs while encouraging migration to structured sections for reliable UI editing.
• Ship an in-plugin UI and server so users can add/manage Script2 accessories without manual JSON editing.

Description

• Updated README.md to document the new recommended configuration shape (on_off_switches and stateless_switches), migration notes, examples, and installation steps.
• Extended config.schema.json to add customUi, customUiPath, a full schema for on_off_switches and stateless_switches, and a conditional devices legacy section for backward compatibility, with improved form metadata and validation rules.
• Added a minimal plugin UI under homebridge-ui/public (index.html, index.js) and a UI server homebridge-ui/server.js that uses @homebridge/plugin-ui-utils to enable the custom config experience.
• Bumped package version and added the UI dependency @homebridge/plugin-ui-utils and ensured node engine requirement is present in package.json.

Testing

• No automated tests were run as part of this change.

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Codex Task