feat: new method to manually add widgets

- Also added a keybind widget as an example for this.
It will not be included in the base jar, but can be manually copied into mods that rely on it.

test-fabric/src/main/resources/assets/modid/lang/en_us.json

@@ -24,5 +24,6 @@
   "modid.midnightconfig.category.lists": "Lists",
   "modid.midnightconfig.category.files": "Files",
   "modid.midnightconfig.category.conditions": "Quiz",
+  "modid.midnightconfig.category.extras": "Extras",
   "modid.midnightconfig.category.multiConditions": "Multi-Conditions"
 }