Sync .gitattributes with pathogen-repo-guide

[victorlin]

Description of proposed changes

This change lets git detect text files. While potentially imperfect, it's better than assuming all files are text.

https://github.com/nextstrain/pathogen-repo-guide/blob/e3bfb52c8155058a3d48592f4268a7382bf3e12a/.gitattributes

Related issue(s)

• https://github.com/nextstrain/pathogen-repo-guide/issues/46

Checklist

• [x] Checks pass

[joverlee521]

👍