Add CheckoutPR.java

[koppor]

Required by JabRef/user-documentation#570

Try it out with

jbang https://github.com/JabRef/jabref/blob/add-checkout-pr/.jbang/CheckoutPR.java 13182

requires jbang

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Otherwise:

1. Users need to authenticate on GitHub
2. Users need to have git installed

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Mandatory checks

• I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [/] Tests created for changes (if applicable)
• [/] Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• [/] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.