Add multiple snail variants.

[aarchangel64]

This PR adds multiple snail types! The code is essentially copied from Vanilla's cat variants. To add a new snail type, the texture should be added to assets/lovely_snails/textures/entity/snail, and an entry that matches the filename should be added to the SNAIL_TYPE enum. Feel free to change, add to, or remove any of the textures that I've included, they're just some basic colour variations that I (and my girlfriend) liked. There's also a special "Moon Snail" that only spawns in full moons (like black cats), making it somewhat rarer. I included this mostly to demonstrate how other special snails could be added - perhaps nether snails might be an idea?

Here are all the new variants that I've included:

No snails were harmed in the making of this commit.

Moon Snail on the left.

omg they're so cute best mod ever