Add set-guard-range sexp

[MjnMixael]

I spent some time looking in to #7025 and it's actually a touch more complex than expected. The AI code limits the guard range with various hardcoded multipliers based on the offending object (weapon, hostile ship, asteroid, etc) and in one instance has an actual hardcoded distance in meters. These values also take in to account the size of the guardee. So it's really pretty complex.

I felt a Guard Tight order would further muddy this up by adding more hardcoded values or multipliers and it wouldn't be easily tunable or readily apparent what the actual difference is for players or mission designers.

After some experimentation I landed on a design that is maybe a touch overly simplistic in some ways but also more configurable. I've added a set-guard-range sexp that works as a clamp for the guard order ranges. The game will use the normal hardcoded logic but if the guard range is > 0 for the guardee ship then it will clamp the calculated range to the guard range. This can be done per-ship which gives mission designers flexibility to tune the guard range based on context. There is not any checking for positive values so you could probably get some pretty interesting AI behavior if the range is set to a value that's too small for the guardee ship class (50m for an Orion might be strange). However, I felt that adding a bunch of dynamic checks was overkill here and mission designers are smart enough to set realistic ranges.

This setup has the advantage of allowing the opposite as well. With set-guard-range you could expand the guard distance if the mission design calls for that sort of thing. Bonus!

Fixes #7025

[wookieejedi]

I realized I should have asked this here just to save any other reviewers the same question:
With the math on the PR looking good I was now looking at where other places that MAX_GUARD_DIST is used, and another are 4 other spots (2 sets of 2 uses). I'm guessing you likely looked at those already, but if not would it make sense to use ai_guard_threshold( there or perhaps not?

[MjnMixael]

I realized I should have asked this here just to save any other reviewers the same question: With the math on the PR looking good I was now looking at where other places that MAX_GUARD_DIST is used, and another are 4 other spots (2 sets of 2 uses). I'm guessing you likely looked at those already, but if not would it make sense to use ai_guard_threshold( there or perhaps not?

I honestly can't remember why I chose to leave those out. I'll need to do some additional testing. I'm going to set this to draft until I can verify.