chore: add ESLint and fix all linting issues

- Add ESLint with TypeScript support
- Configure flat config for ESLint 9
- Fix all unused variable warnings
- Add lint and lint:fix scripts
- Update prepublishOnly to run lint before build
- All 125 tests still passing

Author: jonmatum

eslint.config.js

@@ -0,0 +1,22 @@
+import tseslint from '@typescript-eslint/eslint-plugin';
+import parser from '@typescript-eslint/parser';
+
+export default [
+  {
+    files: ['src/**/*.ts'],
+    languageOptions: {
+      parser,
+      parserOptions: {
+        ecmaVersion: 2022,
+        sourceType: 'module',
+      },
+    },
+    plugins: {
+      '@typescript-eslint': tseslint,
+    },
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
+    },
+  },
+];