feat: pure-Swift AppIntents metadata generator for Linux

[foobisdweik]

This PR introduces a pure-Swift, cross-platform AppIntents metadata generator (AppIntentsGen).

Currently, xtool relies on Xcode's appintentsmetadataprocessor, which restricts building AppIntents to macOS. This fallback parses @AppIntent and @AppShortcutsProvider declarations natively via SwiftSyntax and emits an Apple-compatible JSON schema for extract.actionsdata, allowing apps built on Linux and Windows to expose Shortcuts and Intents to the OS.

Features

• Complete Swift syntax scanner for Intent discovery.
• Cross-platform JSON metadata emitter matching Apple's internal schema.
• Automatic mangled-name encoding and cross-module resolution.
• Integrated into the existing Packer flow as a fallback when Xcode is unavailable.

Tested extensively on Linux (Ubuntu) targeting iOS 17+. Actions are discoverable in the Shortcuts app and support output/magic-variables.