Improve environment variables support

[brillout]

Description

• The docs should be improved.
• Create a new Vike setting env.public.
• How about supporting .env.public files?
• Add import.meta.env support for config files?

[7iomka]

Why my `import.meta.env` is undefined?

[brillout]

@7iomka It's a Vite design limitation: import.meta.env isn't defined in any config file, neither inside Vite's vite.config.js file nor inside Vike's +config.js files.

Because, in order to determine whether the production or development environment variables should be applied, the value of Vite's mode setting needs to be known. And note that mode can be defined inside vite.config.js.

A possibility would be to stop using Vite's mode implemention and have Vike implement its own mode feature. (While we could make import.meta.env work inside +config.js and vike.config.js files, I ain't sure we could make it work inside vite.config.js files: to be researched.)