Replaces move_on_shuttle with a trait (#83550)

## About The Pull Request

`move_on_shuttle` was a variable that existed on `/mob` despite only
ever being used by cameras. This PR downgrades the variable to only
exist on `/mob/camera` types, but have the only thing that variable do
is add a trait that blocks movement on shuttles if not allowed. This
allows us to check the variable without casting to camera, which is what
some code was un-necessarily doing. This also retainst the ability to
add the trait later on to different mobs who we may not want to be on
shuttles that aren't `/mob/camera`.
## Why It's Good For The Game

There's no point for this variable to live on `/mob` if all it can do is
clog up the VV screen, especially if the current utilization for it is
only cameras. Let's move it off `/mob` and turn it into a trait so it
still has the similar utilization on a global level without typecasting
- as well as make it actually work since needless typecasting was
introducing a minor bug
## Changelog
Irrelevant.

---------

Co-authored-by: Time-Green <7501474+Time-Green@users.noreply.github.com>

code/modules/mob/camera/camera.dm

@@ -8,13 +8,16 @@
 	mouse_opacity = MOUSE_OPACITY_TRANSPARENT
 	invisibility = INVISIBILITY_ABSTRACT // No one can see us
 	sight = SEE_SELF
-	move_on_shuttle = FALSE
+	/// Toggles if the camera can move on shuttles
+	var/move_on_shuttle = FALSE
 	/// Toggles if the camera can use emotes
 	var/has_emotes = FALSE
 
 /mob/camera/Initialize(mapload)
 	. = ..()
 	SSpoints_of_interest.make_point_of_interest(src)
+	if(!move_on_shuttle)
+		ADD_TRAIT(src, TRAIT_BLOCK_SHUTTLE_MOVEMENT, INNATE_TRAIT)
 
 /mob/camera/experience_pressure_difference()
 	return