Expose gui.AddCaption

[Denneisk]

https://wiki.facepunch.com/gmod/gui.AddCaption

It's fun to use.

Note, duration of math.huge will break the caption system until the player rejoins. Negative and NaN inputs work like 0 second durations except they only appear if another caption is already displayed, and are relatively harmless.

[thegrb93]

The engine captions command has some build in stuff I don't trust and it's not hard for starfall to render its own captions so probably not gonna add this. May need to reevaluate the addition of it in e2 as well since it can probably be abused.