EXT-1: ExtensionManifest + ExtensionType Models

[ZhuchkaTriplesix]

Epic: #206
Source: docs/04 — Расширения/Система расширений.md

Goal

Create data models for parsing manifest.json.

Implementation

• Create ExtensionManifest class with properties: id, name, version, publisher, type, engines, main, icon, description.
• Create ExtensionType enum (e.g. database_driver, theme).
• Implement fromJson / toJson for serialization.