[feature] add styles exports

[meteorlxy]

What is the problem that this feature enhancement aims to solve?

As the docs said:

The styles and variables can be imported from ../node_modules/data-grid-vue/dist/style.css

It looks a little bit strange.

Describe the solution you'd like

Maybe we can add an extra export for style file, e.g.

{
  "exports": {
    "./style": "./dist/style.css"
  }
}

Then users can import the style from 'data-grid-vue/style'