UI Package Differ Enhancements and Features

## Is Your Feature Request Related To A Problem? Please describe.
Features to add to Package Differ:

* [ ] Line numbers
* [ ] "viewed" button
* [ ] For collapsed diffs, show a loading icon when expanded until the diff is ready. Or, do not show the expanded diff at all utnil the diff is ready. Right now, it seems a bit "laggy" because the timing is not quite right. (the diff is not fetch until expanded, so there is a delay, as expected.)

## Describe The Solution. Why is it needed?
See above.

## Additional Context.
Packages with Diffs:

* https://community.chocolatey.org/packages/mp3tag/3.18.1-beta#diffs

## Related Issues
n/a

┆Issue is synchronized with this [Github issue](https://github.com/chocolatey/home/issues/196) by [Unito](https://www.unito.io)