Add github_pr_tree

[Hacksore]

github_pr_tree lets you view a PR with a file tree on the left. I figured it should be added because it's very handy for code review.

Thank you for contributing! Please confirm this pull request meets the following requirements:

• I followed the contributing guidelines: https://github.com/github/explore/blob/master/CONTRIBUTING.md
• I have no affiliation with the project I am suggesting (as a maintainer, creator, contractor, or employee).

Which change are you proposing?

• Suggesting edits to an existing topic or collection
• Curating a new topic or collection

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Editing an existing topic or collection

I'm suggesting these edits to an existing topic or collection:

• Image (and my file is *.png, square, dimensions 288x288)
• Content (and my changes are in index.md)

[kenyonj]

💯 🍨 Thank you for your contribution! I'll merge this in as soon as CI is 🍏