add optional skylight requirement and allow 0 chance config

[Xalcon]

Greetings! Thank you for this small but genious mod.

There are a lot of automation mods that will trip up when a hefty crop spawns. This PR adds 2 simple changes, both are done in a way that keep the original behavior as the default.

1. Allow 0 weight config for spawns

Previously, the weight was limited to a value between 1 and 100. I've extended the range to include 0 and added a check for 0 in the onCropGrow event handler. The default of 25 is unchanged. This option is targeted towards modpacks which only want to use the hefty crops as decorative objects. It would be up to them to add options to obtain the hefty crops when disabling the spawning.

2. Add sky light requirement

As requested in issue #2, I've added a configuration option that allows hefty crops to only spawn under direct sky light (sky light level == 15). This still allows crops to grow under glas since glas does not change the skylight level, but anything that reduces the skylight level for a given location, would prevent hefty crops to grow. The default value is set to false.