[next] Heroku Support (#2240)

* feat: added env-rewrite support

* feat: added heroku support via app.json

* fix: fixed readme linting error

* fix: fixed error in app.json

* fix: improved environment variables exposed by app.json

src/types/env-rewrite.d.ts

@@ -0,0 +1,3 @@
+declare module "env-rewrite" {
+  export function rewrite(): void;
+}