[B+C+JD] Renamed velocity to direction in World.spawnArrow. Fixes BUKKIT-4838

**The Issue:**
Parameter velocity is actually a direction parameter. Speed is a separate parameter.

**Justification for this PR:**
Ambiguity of the meaning of the velocity parameter, and it is not named as what it actually is.

**PR Breakdown:**
[B+C+JD]: Changes any mention of velocity in `World.spawnArrow` to direction.

**Testing Results and Materials:**
Untested. Each change individually can operate on its own and does not break other components, and is fully backwards/forwards compatible.

**Relevant PR(s):**
B-941 https://github.com/Bukkit/Bukkit/pull/941 - Cross-repository change.
C-1252 https://github.com/Bukkit/CraftBukkit/pull/1252 - Cross-repository change.

**JIRA Ticket:**
BUKKIT-4838 https://bukkit.atlassian.net/browse/BUKKIT-4838