✨ Remove ffmpeg_kit_flutter from the package

[LeGoffMael]

This PR is part of #147.
Will fix duplicate library issues like #37, #129, #153.

[LeGoffMael]

@maRci002 Would this be ok to merge this PR before yours (#147) ?
I would like to release the 3.0.0 without the FFmpeg and with other fixes.

Once the video_thumbnail is ready for web, we could release it as 4.0.0.
(The whole structure of files of the project is actually changed, sorry if it caused a lot of changes on your side)

[maRci002]

@maRci002 Would this be ok to merge this PR before yours (#147) ?

Yes, you can merge this PR. After that, I'm going to update PR #147.

Once the video_thumbnail is ready for web, we could release it as 4.0.0.

I don't think a major version change is necessary. This is because it will only update the example code, modify some underlying painter functionality, and slightly affect the FFmpegVideoEditorConfig and its subclasses.
You are right VideoEditorController will use XFile