Fix building issue on Windows

[matteovivona]

We have this error on CI:

> fastify-socket.io@5.0.0 build D:\a\fastify-socket.io\fastify-socket.io
> tsc --noEmit && tsup-node

CLI Building entry: D:/a/fastify-socket.io/fastify-socket.io/src/index.ts
CLI Using tsconfig: tsconfig.json
CLI tsup v6.7.0
CLI Using tsup config: D:\a\fastify-socket.io\fastify-socket.io\tsup.config.ts
CLI Target: node16
CLI Cleaning output folder
CJS Build start
Error: R] The entry point "D:/a/fastify-socket.io/fastify-socket.io/src/index.ts" cannot be marked as external

CJS Build failed
Error: Build failed with 1 error:
error: The entry point "D:/a/fastify-socket.io/fastify-socket.io/src/index.ts" cannot be marked as external
    at failureErrorWithLog (D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.1[9](https://github.com/ducktors/fastify-socket.io/actions/runs/6236465858/job/16929314826?pr=42#step:8:10)\node_modules\esbuild\lib\main.js:1636:15)
    at D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.19\node_modules\esbuild\lib\main.js:[10](https://github.com/ducktors/fastify-socket.io/actions/runs/6236465858/job/16929314826?pr=42#step:8:11)48:25
    at runOnEndCallbacks (D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.19\node_modules\esbuild\lib\main.js:1471:45)
    at buildResponseToResult (D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.19\node_modules\esbuild\lib\main.js:1046:7)
    at D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.19\node_modules\esbuild\lib\main.js:1075:16
    at responseCallbacks.<computed> (D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.19\node_modules\esbuild\lib\main.js:697:9)
    at handleIncomingPacket (D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.19\node_modules\esbuild\lib\main.js:752:9)
    at Socket.readFromStdout (D:\a\fastify-socket.io\fastify-socket.io\node_modules\.pnpm\esbuild@0.17.19\node_modules\esbuild\lib\main.js:673:7)
    at Socket.emit (node:events:514:28)
    at addChunk (node:internal/streams/readable:343:[12](https://github.com/ducktors/fastify-socket.io/actions/runs/6236465858/job/16929314826?pr=42#step:8:13))
DTS Build start
DTS ⚡️ Build success in 2487ms
DTS dist\index.d.ts [20](https://github.com/ducktors/fastify-socket.io/actions/runs/6236465858/job/16929314826?pr=42#step:8:21)5.00 B
 ELIFECYCLE  Command failed with exit code 1.
Error: Process completed with exit code 1.