Gather all common parser functions into packages/ensnode-utils

[tk-o]

All ours application inside apps/* rely on environment variables to provide configuration. This requires parsing. And it led us to creating very similar functions across all apps, for example, parsing a port number, or a feature flag setting.

We should identify and gather all of those little parsers and put them into a shared package, namely, packages/ensnode-utils. This way, we can reuse some code and not repeat parser implementations (and testing suites) in every app that needs them.