I had to look up some algorithms for the lasso mechanic to work, but it actually wasn't too complicated! I wasn't able to get the mechanic completely bug-free (sometimes it doesn't detect a closed shape), but it works more times than it doesn't 😂
If you're actually curious, a tl;dr is that I generated a Line2D based on the mouse position and then kept checking for a closed loop. Once a closed loop is detected, I used an algorithm to detect if any of the bug positions were within the loop (points in a polygon). I also had to manually check for bug-lasso collisions, because I couldn't figure out how to get Godot to abstract it away for me lol. It was pretty cool once I got it all working!