Add enabled-modes method

[Ambrevar]

trafficstars

modes confusingly only returns the enabled modes. To access the whole mode list, one must use (slot-value ... 'modes).

This has bitten us before. See 20d72d62ccd23b10411193b1404786ebf9f25f84.

I suggest changing modes to return the whole list, and adding an enabled-modes method.