web implementation

[maRci002]

closes #14

The scaling functionality behaves similarly to Android's scales. However, I can update the algorithm to use cropping if both maxWidth and maxHeight are present to match the behavior of iOS.

Running example on the web:

thumbnail.mp4

I have tested it with video_editor package:

video_editor.mp4

I have updated the plugin to use an endorsed federated plugin. You can find more information on this at https://medium.com/flutter/how-to-write-a-flutter-web-plugin-part-2-afdddb69ece6

As for publishing, here are the steps:

• Ensure that the PR can be approved.
• Remove the local path from pubspec.yaml. (I will update the PR.)
• Merge the PR.
• Publish the video_thumbnail_platform_interface as a new package.
• Publish the video_thumbnail_web as a new package.
• Release the new version of the video_thumbnail package.

[TillJohanndeiter]

Nice work 👍

This PR contains the merge of this PR as an open todo. The PR is reviewed and has no conflict. Is there a reason why it's still open?