feat: support user-provided vite plugins

[fnlctrl]

Currently vite plugins defined in .vitepress/config.js are ignored, e.g.

module.exports = {
  vite: {
    plugins: [
      myPlugin() // doesn't work
    ]
  }
}

Therefore, userViteConfig.plugins is appended to createVitePressPlugin result to support custom vite plugins

[lencx]

@fnlctrl I found the solution here #241 (comment)

example: lencx/code-snippets/docs/vite.config.js

[root]
|- [docs]
|	 |- [.vitepress]
|	 |- vite.config.js
|	 `- ...

[fnlctrl]

@lencx Thanks for the workaround! Didn't know it was possible.

The vite option was introduced in 3737b10 so I think plugins defined in it should also work