added functionality to identify environment variables enclosed in ${..} with a regex

This feature enables the yarrrml-parser to read content of a `.env` file and replaces dollar variables with curly braces in a YARRRML document accordingly. Thus the yarrrml-parser follows the common practice to define secrets in a `.env` file. Mapping files will be more reusable as they can be used in environments with different secrets or any other replacable value such as sources, targets etc. Three test cases are provided to test that all variables are replaced and that warnings are emitted when not all variables can be replaced because no environment variable was specified.