Lone Reaver could have terramex targeting

[sprunk]

I got a report/request that Reaver should fire at terramex (with a replay where it would have come in handy https://zero-k.info/Battles/Detail/2062177 ).

According to #4513 Reavers no longer shoot at terramex because they teamkilled each other too much, but (esp. in a 1v1) you often have a lone reaver. Would be nice if it could still fire in "safe" situations. Truly lone Reavers are easy to detect (Spring.GetUnitNearestAlly in radius, ok if nil) so perhaps that's already an improvement that doesn't introduce too much complexity.

[GoogleFrog]

I'm a bit wary of giving units too much situational awareness, as it makes their behaviours harder to learn. Maybe the solution would be a shoot-at-terramex toggle that can be disabled by default for Reaver, and the behaviour could even be added for a few other even more borderline units. Toggles aren't great in general, but this is a pretty hardcore problem so perhaps they're ok in this case. I wouldn't even show people the toggle on the UI by default. The behaviour might currently be tied to the wardfire toggle, which is a bit of an ugly combination, so adding a distinct toggle would be an improvement regardless.

Tbh I might even disable this toggle by default personally, since I haven't gotten much out of the behaviour.