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.

Look at the spell data structure in the repository, likely in JSON files. Find examples of existing fields like 'damage_at_slot_level' to understand the format. Add the new 'casts_at_slot_level' field to the appropriate spells, such as Magic Missile and Eldritch Blast, following the pattern shown in the issue. Verify the changes by checking the JSON structure and ensuring it aligns with the API specification.

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