StructureSpawn should be a class

[seancl]

Currently TypeScript rejects this if condition:

s instanceof StructureSpawn

since StructureSpawn is defined as a type, not a class. However, this works:

s instanceof Spawn

This is somewhat annoying, since the two do seem to be interchangeable. The docs, for what it's worth, specifically refer to the class/prototype as "StructureSpawn" (not "Spawn"): http://support.screeps.com/hc/en-us/articles/205990342-StructureSpawn

[NhanHo]

StructureSpawnwas something added later in the game, it was originally Spawn (that's why both are supported in the game). I kept it as Spawn for backward compatibility, but didn't realize alias type won't allow instanceof.

I've changed StructureSpawn to extends Spawn with no other change. It should fix the instanceof problem but let me know if it causes anything else