Player spawning and respawning related features

Merge !2223 first.

New:

• Added PlayerSpawnPoint(player) hook to Lua
  • Accepted return parameters:
    • x, y, z, angle, spawn_flipped, spawn_on_ceiling, height_is_relative (defaults to true if z was not provided), scale
    • Table containing: all of the above as keys
    • nil to do nothing
• Added playerstarts, matchstarts, and teamstarts[] to Lua
• Added G_FindPlayerStart, G_FindMatchStart, and G_FindTeamStart to Lua
• Added map thing type 36 (Team Spawn Point)
  • String argument 1: Team this player spawn point belongs to

Fixed:

• Nothing to add

Backwards compatible:

• Nothing to add

Backwards incompatible:

• Nothing to add