refactor: extract shared player.ID type and GenerateID to player package

Both game and snake packages had identical PlayerID types and the snake
package imported game.GenerateID. Now both use player.ID and
player.GenerateID from the shared player package.

snake/types.go

@@ -3,6 +3,8 @@ package snake
 import (
 	"encoding/json"
 	"time"
+
+	"github.com/ryanhamamura/c4/player"
 )
 
 type Direction int
@@ -78,10 +80,8 @@ const (
 	StatusFinished
 )
 
-type PlayerID string
-
 type Player struct {
-	ID       PlayerID
+	ID       player.ID
 	UserID   *string
 	Nickname string
 	Slot     int // 0-7