New Item: Shadow Essence/shadow silk

This goes with https://feedback.minecraft.net/hc/en-us/community/posts/360060771051-New-mob-Shadow 

Shadow essence is a drop by Shadows (both types) and can be used to make a new type of potion: the phase potion. However, essence in it's raw form (a black mist-like stuff that you have to keep in a bottle) can't be used, but combining it with string makes Shadow Silk. This item can be used to brew a phase potion.

This potion gives the player the phase effect for 30 seconds, allowing the user to phase through walls one block thick. While there is no level 2 potion, an extended potion (1 minute) does exist and both versions can be crafted into splash and lingering variants, as well as tipped arrows.

Phasing causes half a heart of damage for each entering/exiting of a block (so 1 heart per phase). You cannot phase through missiles, mobs, blocks considered transparent, or blocks that aren't full blocks. Attempting to do so will deal half a heart of damage.

This damage can be negated by using Shadow silk to make Shadow Garb (which has 4 versions--one for each armor slot) a full set of Shadow Garb negates all damage from phasing and attempts. It does not, however, grant the phase effect it just negates the damage from phasing.