Convert `CVAR_*` methods to not use magic strings

Getting the key for a global variable, like the "Skip text" enhancement, is as easy as calling `CVAR_ENHANCEMENT("SkipText")`, but you have to spell `SkipText` correctly every time. Because it's an arbitrary string, failure to do so results in a bug at runtime instead of a build error.

The methods could be made more robust if they're reconfigured to take in constants instead of arbitrary strings (e.g. enum parameters that fetch associated string values from a dictionary).