Calculated Gantt Charts

As referenced by @shahn7 on #9 (closed),

Additionally, one nice to have feature would be if the dates could be partially calculated automatically, if that is possible from the input data. I haven't written a typst package yet, but maybe I could take a stab at an implementation, if that would seem acceptable to you? No promises wrt. timeline, though - so if anyone beats me to it, yay 😄