Add typings for process.env

[brunolemos]

Babel doesn't support namespaces like export namespace App {}, but it does seem to support export declare namespace App {} or extending an existing namescape inside a declaration file (.d.ts) like we are doing here. Everything seems to work fine for this use case. (related)

[Timer]

What about user defined REACT_APP_ vars? Can we fall back to { [key: string]: string | undefined }?
Also, test is a valid env.

[Timer]

Does this merge friendly if the user installs @types/node?

[brunolemos]

What about user defined REACT_APP_ vars? Can we fall back to { [key: string]: string | undefined }? Also, test is a valid env.

Added.

Does this merge friendly if the user installs @types/node?

Yes, just tested and worked as normal. It's because this code is extending an existing type, not creating or replacing completely

[Timer]

I dunno what process.browser is for, we def don't support that. If we do, it's by accident and isn't covered by our semver guarantee. Can we remove that type please?

[brunolemos]

@Timer removed. It's added by webpack, it will probably be always true with cra because there's no ssr support right

[Timer]

@brunolemos ah, yeah, webpack is an implementation detail and we regularly bump major versions of webpack in patches. One day, we might not even use webpack -- so I don't want to rely on this.