feat(build): allow overriding vite config loading

[brc-dd]

By default, Vite will load the vite.config.{js,mjs,ts,mts} from the root. One can disable this behavior by having some config like this:

// .vitepress/config.ts

export default {
  vite: {
    configFile: false
  }
}

One can also pass a string in configFile pointing to some different vite config file (resolved relative to cwd). But it is recommended to use the vite key in vitepress config file instead.

Disabling config auto loading will allow users to keep their vite and vitepress project at the same root.