feat($cli): run debug mode without clearing screen

Author: billyyyyy3320

Diff for: packages/@vuepress/core/lib/node/dev/index.js

@@ -175,7 +175,7 @@ module.exports = class DevProcess extends EventEmitter {
         port: this.port,
         displayHost: this.displayHost,
         publicPath: this.context.base,
-        clearScreen: this.context.options.clearScreen
+        clearScreen: this.context.options.clearScreen || !this.context.options.debug
       }])
 
     config = config.toConfig()

Diff for: packages/docs/docs/api/cli.md

@@ -40,7 +40,7 @@ See [host](../config/README.md#host).
 Open browser when ready.
 
 ### --no-clear-screen
-Do not clear screen when dev server is ready.
+Do not clear screen when dev server is ready. Note that dev server will not clear screen if you start it in debug mode.
 
 ## eject