feat: acid trail for acid cannons

[chaosvolt]

Summary

SUMMARY: Balance "Give acid fired out of a water cannon acid trail effects instead of using emit_actor hackery"

Purpose of change

Some belated fixes for weird behavior with acid used in water cannons. In particular, for whatever reason it likes to only spawn acid at the very edge of its range or in front of any walls it hits, even if it has to shoot acid clean through an enemy it is clearly hitting to do so. I tested this with both the fiddly emit_actor method and with an aoe-type ammo effect and yeah, it doesn't work good either way.

Describe the solution

1. Removed RECOVER_10 from all acid items.
2. Instead, added ammo effects ACID_TRAIL and ACID_TRAIL_WEAK for all acid items that have an acid emit as their drop_action. Idea here is, instead of always just dumping a glob of maximum intensity at the end point of firing, they'll have trails of less intense acid spawn instead.
3. Defined ACID_TRAIL ammo effect, creating a trail of acid between 1 and 3 intensity 66% of the time. Compare with emit_acid_drop always being max intensity at a 100% rate. Used by concentrated acid.
4. Defined ACID_TRAIL_WEAK ammo effect for the other acids that spawn fields to use. Trail spawns intensity of 1 or 2, 33% of the time. Contrast with emit_acid_drop being max intensity but only 50% of the time.

Describe alternatives you've considered

Removing acid cannons because I'm basic instead.

Testing

1. Load-tested in compiled build.
2. Spawned in a fire truck and replaced its water with a jug of concentrated acid.
3. Fired it off, confirmed it left acid only on one tile.
4. Tested firing on a debug critter, observed that acid spawns occur all throughout the line of fire.

A thing I noticed: acid emission from dumping a jug of acid on the ground spreads the field out with overflow to make a big mess, while with the water cannon it always stayed in one tile. emit_actor was being weird when abused via a gun dumping ammo on the ground evidently.

Additional context

8 shots with best available acid:

[scarf005]

haha sulfuric acid goes pew pew