deps: upgrade to ESLint 9 (#70)

eslint.config.mjs

@@ -0,0 +1,54 @@
+import globals from 'globals';
+import tsParser from '@typescript-eslint/parser';
+import eslint from '@eslint/js';
+import jestPlugin from 'eslint-plugin-jest';
+import tseslint from 'typescript-eslint';
+import prettierConfig from 'eslint-config-prettier';
+
+export default tseslint.config(
+  {
+    ignores: [
+      '**/node_modules',
+      '**/dist/',
+      '**/lib/',
+      '**/.nvmrc',
+      'src/fft-api/types/typescript-fetch-client/',
+      'babel.config.js',
+      'jest.config.cjs',
+      'jest.setup.cjs',
+      'eslint.config.mjs',
+      'tsup.config.js',
+    ],
+  },
+  prettierConfig,
+  eslint.configs.recommended,
+  ...tseslint.configs.strict,
+  ...tseslint.configs.stylistic,
+  {
+    files: ['**/*.spec.ts', '**/*.test.ts'],
+    ...jestPlugin.configs['flat/recommended'],
+  },
+  {
+    languageOptions: {
+      globals: {
+        ...globals.jest,
+        ...globals.node,
+      },
+
+      parser: tsParser,
+      ecmaVersion: 2021,
+      sourceType: 'commonjs',
+
+      parserOptions: {
+        project: 'tsconfig.json',
+        tsconfigRootDir: globals.__dirname,
+      },
+    },
+
+    rules: {
+      '@typescript-eslint/no-unused-vars': 'error',
+      'no-undef': 'error',
+      'no-const-assign': 'error',
+    },
+  }
+);