Add number of spells cast field

API issue: https://github.com/5e-bits/5e-srd-api/issues/361

I'd like to propose a new optional field for spells similar to `damage_at_slot_level`: `casts_at_slot_level`. `casts_at_slot_level` holds the value of how many times a specific spell is cast per usage.

For instance, [Magic Missile](https://roll20.net/compendium/dnd5e/Magic%20Missile#content) can be cast 3 times, increasing by 1 for each slot level. An example field for Magic Missile would contain the following:
```json
"casts_at_slot_level": {
"1": 3,
"2": 4,
"3": 5,
"4": 6,
"5": 7,
"6": 8,
"7": 9,
"8": 10,
"9": 11
}
```

Another spell that would contain this field would be [Eldritch Blast](https://roll20.net/compendium/dnd5e/Eldritch%20Blast#content). I'm not sure the extent of spells that would need this field.

I'm not sure if there is a spell that does this, but this may also need to add `casts_at_character_level` if casts increases by character level and not slot level.