Added support for JSON env variables through the "json" feature. Issue #84

[ZocoLini]

Description:
Using the "json" feature the code will validate if the environment variables are a valid JSON string using serde_json when the input starts with '{' and ends with '}'. Additionally, one test has been added to ensure that this validation works correctly with JSON variables and other types of environment variables in the same file.

Related Issue:
Closes #84

[allan2]

Thanks for the PR.

dotenvy is responsible for loading environment variables. Validating a specific format is outside the scope of this library.

My comment on #84 meant that the behaviour of escaping braces should be clearly defined. I have not looked closely at that issue yet. I'm sorry if it seemed to suggest that I would welcome specific format support.

[ZocoLini]

There is no need for escaping braces, the JSON format doesn't start with "${" so the first '{' is pushed into the output string. The thing is, the parse_value() function, after pushing '{' finds a blank space and expecting_end becomes true and then, in the next loop iteration, the function returns an Err. In fact, the environment variable FOO={} is valid for the actual parser but FOO={ } isn't.