Add gg to .gitignore

[koppor]

For interested users, it should be easy to run PRs or the main version. With gg.cmd, this is now very easy:

./gg.cmd gradle :jabgui:run

Works on macOS, Linux, and Windows.

Screenshot from Windows:

Needed to add m2 to .gitignore - eirikb/gg#69

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This is the alternative to #13039 - and I tend to like gg more than mise, because gg does not require any installation, but mise does.

gg.cmd could also be on the PATH, but that would require some additional step by the user, which we want to avoid.

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• Good, because "anyone" can run gradle (without the requirement of JDK installation)
• Bad, because we have a kind of a binary in the repository

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Follow-up-Tasks:

• Update "nerd" hints -> meaning: how to run jabref using gradle without JDK on the command line. "bootstrap"

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.

[koppor]

Users could have gg.cmd on their PATH. eirikb/gg#77. They could also download locally.

Thus the task is to add gg.cmd to .gitignore 😅

[trag-bot]

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