feat: use authored category names for mods

Store categories as authored strings. Trim and deduplicate without changing case, compare without case sensitivity, and keep all observed categories available to filters and editors.

manifest.json

@@ -4,7 +4,7 @@
   "version": "0.1.0",
   "author": "ct",
   "description": "A battle system in the style of classic console RPGs: the party, the enemies and a command menu — Attack, Items, Defend, Escape. The battle of RPG Plus.",
-  "category": "gameplay",
+  "categories": ["gameplay"],
   "sources": [
     "src/**/*.cs"
   ],