Add Support for Linux Platform in flutter_launcher_icons

Hello,

I would like to request the addition of Linux platform support in the flutter_launcher_icons package.

Currently, the package provides functionality for generating launcher icons for Android, iOS, Web, Windows, and macOS platforms, but there is no built-in support for generating Linux icons.

Since Linux is becoming an increasingly important platform for Flutter developers, especially for desktop applications, I believe adding this feature would greatly benefit the community.

Suggested Features:
The ability to automatically generate launcher icons for Linux, similar to how it works for other platforms.
The option to specify icon sizes and paths for Linux in the pubspec.yaml file

This would save a lot of manual work for developers and improve the development experience when targeting the Linux platform.

Thank you for considering this request. I look forward to seeing this feature in future releases!

Best regards,
AmirHossein Azimi

No files, tests, or entry points are named. Start by reviewing how the package generates icons for existing platforms and how pubspec.yaml options are parsed; done means Linux icon generation supports configurable sizes and paths and is covered consistently with the existing platform support.

Written by the indexing model from the issue text.