Enable the `@typescript-eslint/no-unused-vars` eslint rule

[scottanderson]

1. Enable the eslint rule @typescript-eslint/no-unused-vars
2. Work through the various type assertions, replacing them with typesafe alternatives or disabling with appropriate eslint inline config comments.

{
  "rules": {
    "@typescript-eslint/no-unused-vars": ["error", { argsIgnorePattern: "^_" }],
    "no-unused-vars": "off", // @typescript-eslint/no-unused-vars
  }
}