Add option to write back to process.env

The use case where this would be helpful is when developing locally and getting variables from the SSM parameter store that libraries can see via `process.env`. One caveat is that env-ssm would need to load them first before the library code ran, but that's usually the case anyway.

The issue names no files, tests, or entry points. Review the package's public configuration and parameter-loading flow first, then determine where an opt-in write-back option belongs and how loading must precede consumers. Done means retrieved SSM values are available through process.env when enabled without changing the default behavior.

Written by the indexing model from the issue text.