Add option to only parse $ALL_CAPS env vars

Especially for nginx configs, lots of directives use $lowercase_variable, so having to escape a lot of content.

It would be nice to have an caps-only flag to only detect env vars that are uppercase.

Look at the parsing logic in the main package to understand how environment variables are currently detected. The change involves adding a flag to filter matches to only uppercase variables like $ALL_CAPS. Check existing tests for substitution behavior to add a test for the new flag. Running the CLI with sample nginx configs will verify the flag works as intended.

Written by the indexing model from the issue text.