Lint for /var/ in proc args

[ike709]

Lints for this BYOND bug: https://www.byond.com/forum/post/2830750

The following code prints 5 in BYOND but errors with Unknown identifier "global.bar" in OpenDream.

/datum/proc/foo(/var/bar = 5)
    return

/proc/main()
    world.log << global.bar // This prints 5 in BYOND

This PR doesn't replicate BYOND behavior, it just adds a lint. Ref #2065 for tracking the status of replicating BYOND behavior.

Credit to @out-of-phaze for reminding me of this BYOND bug's existence.

I tested about half a dozen codebases and only found a single hit in goonstation.

[ZeWaka]

fixed on goon