Updated chewie player example for pub.dev

[zyllus17]

Updated the main.dart file with a simpler example and null safety to start working with Chewie player.

Reason for creating this example - Issue #697

If approved this will fix the example mentioned in Issue #697

[diegotori]

This is not gonna fly in its current form, simply because it will override the more comprehensive example that already exists, which should be retained.

If you want to keep both, you have to incorporate the existing demo app into this PR. Otherwise, we're essentially wiping out useful code in this PR.

[zyllus17]

I can create a new file with a name chewie_video_player.dart and place the widget in there but then it won't fix the issue of showing the correct example in the pub.dev. Will it be possible to point to a different file, prefereably app.dart from your end so that the correct example can be shown on pub.dev