[Refactor] remove ffmpeg_kit dependency and add web implementation

[maRci002]

closes #146

This PR contains the following:

• Removed the dependency on the ffmpeg_kit package.
• Created FFmpegConfig to improve video and cover export functionality.
• Added an example demonstrating how to export video and cover images using either ffmpeg_kit or ffmpeg_wasm, depending on the platform.
• Updated the video_thumbnail version to 0.6.0.
• Updated the project to work on the web.
• Updated the documentation to reflect these changes.

Breaking changes

• Replaced File with XFile in VideoEditorController.file.
• Removed VideoEditorController.getMetaData.
• Removed VideoEditorController.exportVideo.
• Removed VideoEditorController.extractCover.

Still need:

• [x] Merge PR for redleafsofts/flutter_ffmpeg_wasm#1
• [ ] Merge PR for justsoft/video_thumbnail#135
• [ ] Remove git and local path from pubspec.yaml, this needs the Pull Requests to be merged.
• [x] Update documentation how to create custom export, especially the executeFFmpegWeb and executeFFmpegIO methods in the example.
• [x] When CropScreen is closed the VideoPlayer is black
  • Problem: flutter/flutter#124210 using two VideoPlayer at the same time: CropGridViewer.preview / CropGridViewer.edit
  • Solution: since we never show two video players side by side at the same time, the VideoPlayer key must change as soon as we are done cropping
• [x] Check why cropStyle: const CropGridStyle(background: Colors.transparent), is needed on the web. It seems that Path.combine breaks on the web.
  • HTML Renderer: flutter/flutter#44572
  • Canvaskit renderer: flutter/flutter#124675
  • Solution: Set Path's fillType to PathFillType.evenOdd

Please refer to the redleafsofts/flutter_ffmpeg_wasm#1 README for instructions on how to run ffmpeg_wasm on the web.

This is what it looks like on the web (using HTML renderer):

https://user-images.githubusercontent.com/8436039/231599361-de6faade-c7ed-4514-86e0-7cfce1f6b195.mp4