[std/dotenv] - change .env.example to .env.required

[cknight]

The dotenv module has the concept of a .env.example file which dictates a set of environment variables which must be present (either through .env/.env.default files or via the process environment). If any are missing after the dotenv is loaded then an exception is thrown. A secondary use case is that it can be used to document the purpose of these environment variables. I propose renaming .env.example to .env.required to be clearer as to its purpose. 'Example' does not convey that the entries are required to exist.