Add clock.time() method

Apparently there is no timer functionality on pgzero, for instance, to record how fast was player 1 to finish the level/game or even time played in general.

I am not sure about where to put this timer, but I guess that it could be added to the Actor class I guess...

Start by reviewing the existing clock API and the Actor class, which the issue identifies as possible locations for timing functionality. Clarify the intended clock.time() behavior and placement before implementing; done should include a usable way to measure elapsed gameplay or level-completion time.

Written by the indexing model from the issue text.