Dragon tags & mechs

[heypr]

• Requested by Josh65.
• Adds 3 paper-only, 1.20+ tags:
  • gateway_count
  • healing_crystals
  • respawn_crystals
• Adds 1 paper-only, 1.20+ mechanism:
  • spawn_gateway
• Adds 1 regular tag:
  • first_dragon_killed
• Adds 4 regular mechanisms;
  • respawn_dragon
  • reset_crystals
  • respawn_phase
  • first_dragon_killed

I opened this as a draft PR because I haven't tested any of this/just wrote this up, and want it open so that I don't lose it in my sea of local and published branches. Feel free to comment/review though!