feat: add native Flutter debug adapter and schemas

[AkhmadRamadani]

Summary

This PR enables native Flutter DAP debugging directly within the official Dart extension.

What's Changed

• Registered Debug Adapters: Added flutter and Flutter debug adapters in extension.toml.
• Flutter Configuration Schema: Added debug_adapter_schemas/flutter.json supporting program, deviceId, flutterMode, toolArgs, flutterPath, useFvm, etc.
• Smart Defaults & Inferences:
  • Automatically defaults debug_mode to flutter when using adapter "flutter" or "Flutter", eliminating the need to explicitly specify "type": "flutter".
  • Defaults request to "launch" if omitted.
  • Adds support for custom Flutter path via flutterPath / flutter_path.
  • Added automatic fallback to FVM (fvm flutter debug_adapter) if flutter is not in PATH but fvm is.
• Documentation: Added Flutter & Dart debugging documentation with configuration examples in README.md.

Example .zed/debug.json

[
  {
    "adapter": "flutter",
    "label": "Launch Flutter App",
    "program": "lib/main.dart",
    "deviceId": "macos"
  }
]

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[AkhmadRamadani]

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