Added can_grow function to plant definition (#3131)

game_api.txt

@@ -382,6 +382,8 @@ The farming API allows you to easily register plants and hoes.
 		-- ^ Always provide a plant texture for each step, format: modname_plantname_i.png (i = stepnumber)
 		minlight = 13,                         -- Minimum light to grow
 		maxlight = default.LIGHT_MAX           -- Maximum light to grow
+		can_grow = function(pos)               -- Сalled every growth tick to check if the plant can grow, returns bool
+		                                       -- (optional, checks for wet soil by default)
 	}