Add syntax for coordinates relative to the bottom of the world

With worlds in 1.18 starting at y = -64 and custom worlds allowing for a custom minimum world height, map makers can no longer reliably assume that y = 0 is the bottom of the world. Thus to remedy this I suggest adding a new coordinate syntax to allow mapmakers to specify y-coordinates relative to the bottom of the world, and also relative to the top of the world. For instance, the backtick could be used to specify relative to the bottom of the world, and the double backtick to specify relative to the top of the world. These coordinates would be interchangeable with relative and absolute coordinates, but not local coordinates.

Usage examples:

#teleport self to top of world

/tp @s ~ ｀｀ ~

# fills stone from the bottom of the world to one block below the current position

/fill ~ ｀ ~ ~ ~-1 ~ stone

#checks if the executor is within 1 block of the top of the world

/execute positioned ~ ｀｀ ~ if entity @s[distance=..1]