New biome, Sandy Seafloor!

I've been developping the look and environment of the Sandy Seafloor biome, I may tweak it a bit still.

There's a bunch of new stuff : -a new biome environment, sandy dunes and caves under the sea held by spiral shell pillars. You start to play with the sand blocks and starfish switches in there.

-added dash shrimp enemy

-added bubble clam enemy (not in any level yet)

-added starfish switches and starfish blocks, which activate and deactivate when you dash on the switch (not in any level yet)

-added a new level, Sand Digger, to introduce the sand blocks mechanic, and it's playable in the demo alongside the other bonus levels!

Plus some tweaks to Follow that chest! and to the blue canon squids to feel a bit more dangerous.

For the next update I will probably pump a bunch of levels into this new biome to make it exist, stay tuned.