Add Hangar (PaperMC) upload step to create-release pipeline

[Copilot]

The release pipeline only published to GitHub Releases; it now also uploads to hangar.papermc.io via benwoo1110/hangar-upload-action@v1.

Changes

• New step: "Upload to Hangar" — appended after the GitHub release asset upload in create-release.yml
  • Uploads the universal JAR targeting PAPER, VELOCITY, and WATERFALL platforms
  • Reuses the already-computed VERSION env var and changelog step output for version/description
  • Credential supplied via ${{ secrets.HANGAR_API_TOKEN }} (placeholder — secret must be added to repo settings)
  • slug set to AntiVPN — update if the Hangar project slug differs

- name: Upload to Hangar
  uses: benwoo1110/hangar-upload-action@v1
  with:
    api_token: ${{ secrets.HANGAR_API_TOKEN }}
    slug: AntiVPN
    version: ${{ env.VERSION }}
    channel: release
    files: |
      [
        {
          "path": "./build/libs/AntiVPN-${{ env.VERSION }}-universal.jar",
          "platforms": ["PAPER", "VELOCITY", "WATERFALL"]
        }
      ]
    description: ${{ steps.changelog.outputs.content }}