fix: Add server property of FtpServer to type definition

ftp-srv.d.ts

@@ -1,3 +1,4 @@
+import type { Server } from 'node:net'
 const EventEmitter = import('events').EventEmitter
 
 export class FileSystem {
@@ -120,6 +121,7 @@ export interface FtpServerOptions {
 }
 
 export class FtpServer extends EventEmitter {
+  server: Server
   constructor(options?: FtpServerOptions)
 
   readonly isTLS: boolean