Fix CI build and ESLint configuration

- Add type declarations for @checkend/browser and @checkend/node
- Move SDK packages to optional peer dependencies
- Add ESLint flat config for ESLint 9
- Fix unused import in testing.ts

Author: geetfun

eslint.config.js

@@ -0,0 +1,17 @@
+import js from '@eslint/js'
+import tseslint from 'typescript-eslint'
+
+export default tseslint.config(
+  js.configs.recommended,
+  ...tseslint.configs.recommended,
+  {
+    ignores: ['dist/**', 'node_modules/**'],
+  },
+  {
+    files: ['**/*.ts', '**/*.tsx'],
+    rules: {
+      '@typescript-eslint/no-explicit-any': 'off',
+      '@typescript-eslint/no-unused-vars': ['error', { argsIgnorePattern: '^_' }],
+    },
+  }
+)