feat: replace spell category by spell groups

This also allows to assign a spell to multiple spell groups, which is the case for many spells in
the SRD.

Additionally, this makes many small improvements and fixes to the provided spell compendium.

tools/json-pack-tools.js

@@ -57,7 +57,7 @@ export async function convertJSONToPack(contents) {
 /**
  * Converts a pack file (NeDB) to a JSON string.
  * @param {string} filename The name of the pack file
- * @returns {Promise<Array<unknown>>} A promise that resolves to an array of the documents in the pack file
+ * @returns {Promise<string>} A promise that resolves to an array of the documents in the pack file
  */
 function convertPackFileToJSON(filename) {
     const db = new Datastore({ filename, autoload: true });