[Feature]: Hope to add a configuration to support loading .env files with custom names.

[targeral]

What problem does this feature solve?

I have two files, .env and .dev.env, and I wish for .dev.env to take effect during local development, while .env should be active in the production environment.

What does the proposed API look like?

export default defineConfig({
   buildConfig: {
     envFileName: ['.dev.env'],
   }
});