Add Syncthing's .gitignore files

[hollowaykeanho]

Description

The Syncthing (https://docs.syncthing.net/) peer-to-peer network sharing application is quite commonly used in 2024 for decentralized networking. Unfortunately, it also introduces a bunch of its own hidden files that can be tracked by git.

Expected Behavior

artifacts are added into .gitignore file.

Current Behavior

.gitignore is not tracking Syncthing.

Associated Data Files

No response

[hollowaykeanho]

implemneted in 10ce583