Add Linux AppImage build to workflow, update packages and replace file_selector and lints with their successors, Fix Lint warnings

[prateekmedia]

• This add linux appimage to github workflow
• Also I have removed the test.dart files as they do nothing and contain nothing important, also I deleted linux_file_picker.dart as it's not needed now due to file_picker plugin
• Other changes are mentioned on the commit message.

[Hexer10]

Thanks for the nice pr!

I'll merge as it is, the only thing I'm going to revert back the the flutter_lints package. I'm aware that it is now the standard, but I'm more used the lint and I prefer it since it's stricted that the offical package.