[TF2] [VScript] Expose `engine->GetEntityCount()`

[Marioiscool246]

Since TF2 has a low edict count compared to other modern source games, having access to this to function is useful when creating entities with vscript. For example, spawning non essential entities through vscript for the purpose of effects such as sparks, decals, or something like trigger_particle can be skipped if there's already too many edicts that are active, preventing a server crash.

[worMatty]

Great idea.