chore($core): deprecate official i18n-ui plugin due to its low availability

Author: ulivz

Diff for: packages/@vuepress/plugin-i18n-ui/.npmignore

@@ -65,7 +65,6 @@ module.exports = ctx => ({
     }
   },
   plugins: [
-    ['@vuepress/i18n-ui', !ctx.isProd],
     ['@vuepress/back-to-top', true],
     ['@vuepress/pwa', {
       serviceWorker: true,

Diff for: packages/@vuepress/plugin-i18n-ui/README.md

@@ -166,23 +166,23 @@ Since all plugins with the same name can be applied ONLY once by default, users
 // theme/index.js
 module.exports = {
   plugins: [
-    '@vuepress/i18n-ui',
-    { route: '/i18n-page/' }
-  ] 
+    'vuepress-plugin-xxx',
+    { name: 'foo' }
+  ]
 }
 ```
 
 ```js
 // .vuepress/config.js
 module.exports = {
   plugins: [
-    '@vuepress/i18n-ui',
-    { route: '/i18n/' }
+    'vuepress-plugin-xxx',
+    { name: 'bar' }
   ] 
 }
 ```
 
-Then the final route of i18n UI is `/i18n/`.
+Then the final value of `name` option will be `bar`.
 
 
 ## Others

Diff for: packages/@vuepress/plugin-i18n-ui/client.js

@@ -4,8 +4,7 @@ Plugins usually add global-level functionality to VuePress. There is no strictly
 
 1. Extend the page's metadata generated at compile time. e.g. [@vuepress/plugin-last-updated](./official/plugin-last-updated.md);
 2. Generate extra files before or after compilation. e.g. [@vuepress/plugin-pwa](./official/plugin-pwa.md);
-3. Add extra pages. e.g. [@vuepress/plugin-i18n-ui](./official/plugin-i18n-ui.md);
-4. Inject global UI. e.g. [@vuepress/plugin-back-to-top](./official/plugin-back-to-top.md);
-5. Exntend command of CLI，如 [vuepress-plugin-export](https://github.com/ulivz/vuepress-plugin-export)。
+3. Inject global UI. e.g. [@vuepress/plugin-back-to-top](./official/plugin-back-to-top.md);
+4. Exntend command of CLI，如 [vuepress-plugin-export](https://github.com/ulivz/vuepress-plugin-export)。
 
 ![Architecture of VuePress](/architecture.png)

Diff for: packages/@vuepress/plugin-i18n-ui/index.js

@@ -167,23 +167,23 @@ $accentColor = #f00
 // theme/index.js
 module.exports = {
   plugins: [
-    '@vuepress/i18n-ui',
-    { route: '/i18n-page/' }
-  ] 
+    'vuepress-plugin-xxx',
+    { name: 'foo' }
+  ]
 }
 ```
 
 ```js
 // .vuepress/config.js
 module.exports = {
   plugins: [
-    '@vuepress/i18n-ui',
-    { route: '/i18n/' }
+    'vuepress-plugin-xxx',
+    { name: 'bar' }
   ] 
 }
 ```
 
-i18n UI 最终的路由将是 `/i18n/`.
+name 的最终值将是 `bar`.
 
 ## 其他
 

Diff for: packages/@vuepress/plugin-i18n-ui/index.vue

@@ -4,8 +4,7 @@
 
 1. 拓展在编译期生成的页面元数据，如：[@vuepress/plugin-last-updated](./official/plugin-last-updated.md)；
 2. 在编译前后生成额外的文件，如：[@vuepress/plugin-pwa](./official/plugin-pwa.md)；
-3. 增加额外的页面，如：[@vuepress/plugin-i18n-ui](./official/plugin-i18n-ui.md)；
-4. 注入全局的 UI, 如：[@vuepress/plugin-back-to-top](./official/plugin-back-to-top.md);
-5. 拓展 CLI 的指令，如 [vuepress-plugin-export](https://github.com/ulivz/vuepress-plugin-export)。
+3. 注入全局的 UI, 如：[@vuepress/plugin-back-to-top](./official/plugin-back-to-top.md);
+4. 拓展 CLI 的指令，如 [vuepress-plugin-export](https://github.com/ulivz/vuepress-plugin-export)。
 
 ![Architecture of VuePress](/architecture.png)

Diff for: packages/@vuepress/plugin-i18n-ui/package.json

@@ -27,7 +27,6 @@
   "devDependencies": {
     "@vuepress/plugin-back-to-top": "^1.0.0-alpha.44",
     "@vuepress/plugin-google-analytics": "^1.0.0-alpha.44",
-    "@vuepress/plugin-i18n-ui": "^1.0.0-alpha.44",
     "@vuepress/plugin-medium-zoom": "^1.0.0-alpha.44",
     "@vuepress/plugin-pwa": "^1.0.0-alpha.44",
     "@vuepress/theme-vue": "^1.0.0-alpha.44",