[My Sweet Cataclysm] Add Gum Spiderlings

[TheSaddestGoomba]

Summary

Content "Gum spiders now spawn as new baby versions."

Purpose of change

-Currently gum spiders just pop-up fully formed, sometimes adjacent to you and already hostile. This eases that player pain-point by having them spawn as spiderlings from rotten gum before maturing into an actual threat.
-This also sets up a 'life'cycle for the newly animate gum spiders.

Describe the solution

Add two new monsters:
-gum spiderling
-caffeinated gum spiderling
Hook them into the existing monsters and egg groups so that they progress:
-> Gum -> Spiderling -> Spider -> Gum ->

Describe alternatives you've considered

Also setting up copy_from for the caffeinated variants but this was such a simple addition and I wanted to keep it that way.
Maybe a future PR to audit the gum spiders. Maybe alongside setting up the caffeinated versions to inject you with actual caffeine, and giving nicotine gum its own nasty variant.

Testing

Loaded up a new world adding only My_Sweet_Cataclysm. Debug time shifted and teleported to test spawning of spiderlings from item drops, upgrade into adults, 'laying' of gum, and hatching.

Additional context

[TheSaddestGoomba]

zed_amphibian.json is an unintended carry-over. Will toss the stowaway this evening.