【Feature Request】public directory customization should be set in one place

[magentaqin]

1、status quo

If I want to customize public dir, I have to set it both in rollup.config.js and package.json, which is pretty tedious and less friendly to new comers.
First，pass watch param to livereload plugin:

Second, change sirv public dir:

2、Expect

Set public path only in rollup.config.js and work out through.

If possible, I will create a PR to enhance it.