Add envRoot parameter

[cawa-93]

There are a number of scenarios when .env files location and vite-root must be different:

• Frontend with Vite is only part of a larger project and is located in one of the subdirectories.
• Monorepo with a few roots. But the .env files are common.

I feel the need to specify in the configuration where to look for an .env files:

I propose add envRoot parameter

// vite.config.js
{
  root: __dirname,
  envRoot: '../../some/where/'
}

interface UserConfig {
  /**
   * The directory in which the `.env` files will be searched. 
   * Set to `false` to disable `.env` files
   * @default Project root
   */
  envRoot?: string | false
}

And inlineConfig.envFile will be deprecated

// config.ts

function resolveConfig() {
  // load .env files
  const userEnv = inlineConfig.envRoot !== false && loadEnv(mode, inlineConfig.envRoot || resolvedRoot)
}

As a workaround, at the moment, you have to use loadEnv from vite to load .env files and set the values in process.env manually.