[2.0 Beta] Bring back env and mode in config

[AlexandreBonaventure]

Is your feature request related to a problem? Please describe.
Migrating from v1, I realized env + mode options in config are gone in v2.

Describe the solution you'd like
Not sure about the reasoning here, but we've found these options to be very helpful. We share common configuration between different projects in a monorepo, and the ability to override these in the config allows more flexibilty for us, and let us handle it in a more programmatic fashion (rather than creating env files). It can probably be reimplemented through a custom user plugin but that would mean reimplementing the define plugin.

[AlexandreBonaventure]

happy to submit a PR btw

[yyx990803]

396bbf8 allows mode to be specified via user config.

As of beta.10 you can also directly set process.env.VITE_FOO = 'hi' in the config file.