refactor(config): Export loadEnv() function

[cawa-93]

For some projects, such as electron-vue-next, there is a need to load environment variables from non-standard locations and to transfer these environments to other builders (such as rollup, etc.).

This PR will allow simply import loadEnv function from vite and use its algorithm instead of duplicating this algorithm in your own code.

Use cases

Configuring the building of two bundles using a single mechanism for loading environment variables.

import { loadEnv } from 'vite';


// The location of env files can be arbitrary
const dotenvLocation = path.resolve(__dirname, '../../')


// Load all environment variables with the prefix VITE_ to build a web interface
const envForRenderer = loadEnv('production', dotenvLocation)
buildRendererProcessWithEnv(envForRenderer)


// Load all environment variables with the prefix ELECTRON_ to build a backend
const envForMain = loadEnv('production', dotenvLocation, 'ELECTRON_')
buildMainProcessWithEnv(envForMain)

Another use case:
The development environment can be configured to automatically generate *.d.ts files based on .env* and describing environment variables and their types for each mode.

[aleclarson]

The loadEnv function mutates process.env. We should separate the mutation into its own function, so loadEnv can be called without side effects.

[cawa-93]

I split loadEnv function into two:

• loadEnv - reads files and returns an object with environment variables. Without side effects.
• expandEnv - mutate process.env. For backward compatibility, I call the expandEnv wherever loadEnv is called. But, I assume that expandEnv is not needed everywhere, and its calls can be reduced or completely removed.

[aleclarson]

I don't have publish rights, so you'll have to wait for @yyx990803. Hopefully we can get auto-release configured once v1.0.0 is released.