support --dotenv for nuxi dev and nuxi build

[NeroThroN]

Describe the feature

On Nuxt2 there was a --dotenv to specify a different .env file in production, staging or development environments.

On Nuxt3 only the .env at the root directory is used and there is no configuration to use another file.

Nuxt2 doc
Nuxt3 doc

Additional information

• Would you be willing to help implement this feature?
• Could this feature be implemented as a module?

Final checks

• Read the contribution guide.
• Check existing discussions and issues.

[danielroe]

Merged in nuxt/framework#7660.

[HenoTaMyJIb]

Can --dontenv support also be added to nuxi generate?

[danielroe]

Makes sense 👍