feat(fetch): plugin system + eslint/tsconfig migration

- Add fetch plugin API (definePlugin, plugins) with type-level option flow.
- Migrate to eslint flat config and composite tsconfig.

core/fetch/tsdown.config.ts

@@ -3,5 +3,6 @@ import { sharedConfig } from '@robonen/tsdown';
 
 export default defineConfig({
   ...sharedConfig,
+  tsconfig: './tsconfig.src.json',
   entry: ['src/index.ts'],
 });