Make unused variables an ESLint error, not just a warning (#568)

We don't have any unused variables, nor do we want to have them, but
keeping this as a warning means we'll occasionally commit code with
unused variables. This is quite annoying in GitHub UI that shows
warnings in the list of changed files in every PR even for files that
weren't changed as a part of the PR.

Author: Pookie717

.eslintrc.json

@@ -41,6 +41,7 @@
 				"@typescript-eslint/no-explicit-any": 0,
 				"@typescript-eslint/no-empty-interface": 0,
 				"@typescript-eslint/no-empty-function": 0,
+				"@typescript-eslint/no-unused-vars": "error",
 				"no-constant-condition": 0
 			}
 		},