feat: (re)add GoTomato.go

- refactor server package
  - update package name
  - rename `StartServer()` to `Start()`
- (re)add `GoTomato.go` to call `server.Start()`
- update README

cmd/server/main.go

@@ -1,4 +1,4 @@
-package main
+package server
 
 import (
 	"git.smsvc.net/pomodoro/GoTomato/internal/websocket"
@@ -6,7 +6,7 @@ import (
 	"net/http"
 )
 
-func main() {
+func Start() {
 	http.HandleFunc("/ws", websocket.HandleConnections)
 
 	log.Println("Pomodoro WebSocket server started on :8080")