[Feature Request] Improve TypeScript support for config file.

[ulivz]

Is your feature request related to a problem? Please describe.

Make configuration easier.

Describe the solution you'd like

Same as vite :

import { defineConfig } from 'vitepress'

export default defineConfig({
   lang: 'en-US',
   // ...
}) 

But we need to improve the support for theme config:

By default, defineConfig helper leverages the theme config type from default theme:

import { defineConfig } from 'vitepress'

export default defineConfig({
  themeConfig: {
     // Type is `DefaultTheme.Config` 
  }
})

If you use a custom theme, you'll be able to pass the generics type for your custom theme, and you need overload it with the second parameter of defineConfig helper:

import { defineConfig } from 'vitepress'
import { ThemeConfig } from 'your-theme'

export default defineConfig<ThemeConfig>({
  themeConfig: {
    // Type is `ThemeConfig` 
  }
}, true); // declare `usingCustomTheme` and discard usage of the default theme.

Describe alternatives you've considered

No response

Additional context

No response

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