refactor(server): separate server from app

我們需要 NCM API 的伺服器部分，但 app.js
目前的做法讓我們不太好單獨啟動伺服器。

我將 app.js 的伺服器部分抽成各個函數，並將原有
會 blocking 的函數更改為非同步函式。
這樣不僅能最大化善用 Node.js 的 Event Loop，
亦能提升未來維護的程式碼易讀性。

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "NeteaseCloudMusicApi",
-  "version": "4.2.0",
+  "version": "4.3.0",
   "lockfileVersion": 2,
   "requires": true,
   "packages": {
     "": {
       "name": "NeteaseCloudMusicApi",
-      "version": "4.2.0",
+      "version": "4.3.0",
       "license": "MIT",
       "dependencies": {
         "axios": "^0.24.0",
@@ -1502,6 +1502,8 @@
       "resolved": "https://registry.npm.taobao.org/enquirer/download/enquirer-2.3.6.tgz?cache=0&sync_timestamp=1593693291943&other_urls=https%3A%2F%2Fregistry.npm.taobao.org%2Fenquirer%2Fdownload%2Fenquirer-2.3.6.tgz",
       "integrity": "sha1-Kn/l3WNKHkElqXXsmU/1RW3Dc00=",
       "dev": true,
+      "optional": true,
+      "peer": true,
       "dependencies": {
         "ansi-colors": "^4.1.1"
       },
@@ -6743,6 +6745,8 @@
       "resolved": "https://registry.npm.taobao.org/enquirer/download/enquirer-2.3.6.tgz?cache=0&sync_timestamp=1593693291943&other_urls=https%3A%2F%2Fregistry.npm.taobao.org%2Fenquirer%2Fdownload%2Fenquirer-2.3.6.tgz",
       "integrity": "sha1-Kn/l3WNKHkElqXXsmU/1RW3Dc00=",
       "dev": true,
+      "optional": true,
+      "peer": true,
       "requires": {
         "ansi-colors": "^4.1.1"
       }