Enhance is_detector to include OBSERVER type

[chadspratt]

observers are always ready, but build_progress == 0 unless they are detected.

I considered changing is_ready but build_progress can be < 1 if a mothership is cloaking a warp-in so it would need to specify all the non-warpin units for that to work. That would be more correct for immortals/disruptors/etc that are always ready if they exist but it seemed more fraught.

[BurnySc2]

Perhaps a better suggestion would be to split the set of detectors into mobile detectors, static detectors (here: buildings, not OBSERVERSIEGE), then check

1. Is it a mobile detector?
2. Is it a static detector and is_ready (building case, might be invisible if cloaked by mothership)
3. Is it a photoncannon and is_powered (and also is_ready)

[chadspratt]

As far as i know, the only detectors that can be cloaked are the observer and a photon cannon via mothership. A cloaked photon cannon could be unfinished or unpowered, so the only unit that is guaranteed to be a ready detector is the observer.