Test mark as viewed

[alexr00]

Refs: #3287

• anyOS @rzhao271
• anyOS @mjbvz

Complexity: 3

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The old UX for marking a file in a PR as viewed caused some button jumping and used icons that were hard to understand. Within the limitations of the tree view, we've tried to improve upon this.

1. Install the pre-release version of GitHub Pull Requests and Issues (instructions: https://github.com/microsoft/vscode-pull-request-github/wiki/Pre-release-Build). Browser is fine if you want.
2. Open a PR that you don't mind marking files as viewed in (You can open a PR from the "Pull Requests" view).
3. Figure out how to mark a file as viewed. Verify that the icons representing "mark as viewed" and "mark as unviewed" make sense.
4. Verify that when a file has been marked as viewed that it has no color and the git file decoration changes into a check.
5. Verify that that when you mark a file as unviewed it regains it's color and file decoration.

[mjbvz]

Test on MacOS and seems to work well

To improve discoverability, could there also be a button in the editor tool bar that shows in diffs from a PR?