Add Modrinth publish workflow for GitHub Actions

.github/.gitignore

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+.DS_Store

.github/workflows/modrinth.yml

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+name: Modrinth-Release
+on:
+  release:
+    types: [published]
+steps:
+      - uses: Kir-Antipov/mc-publish@v3.3
+        with:
+          # Publish on Modrinth.
+          modrinth-id: bskyblock
+          modrinth-token: ${{ secrets.MODRINTH_TOKEN }}