chore: prepare package for npm publish

Add LICENSE (MIT), README, .npmignore. Update package.json metadata
and tsconfig module resolution.

README.md

@@ -0,0 +1,33 @@
+# vite-plugin-port-checker
+
+A Vite plugin that warns when the dev server listens on a port blocked by browsers (e.g. Chrome, Firefox).
+
+Reference: [Fetch spec — port blocking](https://fetch.spec.whatwg.org/#port-blocking)
+
+## Install
+
+```bash
+npm install vite-plugin-port-checker -D
+# or
+pnpm add vite-plugin-port-checker -D
+# or
+yarn add vite-plugin-port-checker -D
+```
+
+## Usage
+
+```ts
+// vite.config.ts
+import { defineConfig } from "vite"
+import checkRestrictedPort from "vite-plugin-port-checker"
+
+export default defineConfig({
+  plugins: [checkRestrictedPort()],
+})
+```
+
+When the dev server starts on a restricted port, you'll see a yellow warning in the console suggesting you change `server.port`.
+
+## License
+
+MIT