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/tsconfig.src.json

@@ -0,0 +1,9 @@
+{
+  "extends": "@robonen/tsconfig/tsconfig.base.json",
+  "compilerOptions": {
+    "composite": true,
+    "types": ["node"],
+    "tsBuildInfoFile": "./node_modules/.tmp/tsconfig.src.tsbuildinfo"
+  },
+  "include": ["src/**/*.ts"]
+}