Add `import.meta.envName` for build EnvName

Describe the feature

Describe the feature

The current implementation of envName in Nuxt allows for custom environment names (e.g., staging) to be defined at build time, as per the documentation https://nuxt.com/docs/api/commands/build#options & https://nuxt.com/docs/getting-started/configuration#environment-overrides. However, there is no straightforward way to access this custom envName at runtime.

Proposal:
Introduce import.meta.envName to reflect the custom envName defined during the build process. By default, it would to return production, development, or test, but when a custom envName is provided, it would return the defined value (e.g., staging).

Use Cases:

1. Styling and Customization:

   if (import.meta.envName === 'staging') {
       setLogoColor('blue');
   }
   

2. Third-Party Module Configuration:

   import * as Sentry from '@sentry/browser';
   
   Sentry.init({
       dsn: 'https://example@sentry.io/12345',
       environment: import.meta.envName, // Pass environment, 'staging' for example
   });
   

Additional information

• Would you be willing to help implement this feature?

Start with the Nuxt build-option and environment-overrides documentation linked in the issue, then trace Nitro's existing build-time envName handling and runtime entry points. Done means import.meta.envName exposes production, development, and test by default, while returning a custom value such as staging when one is configured.