feat: allow $dotenv tags in variables

This is essentially the same change as in this commit https://github.com/rest-nvim/rest.nvim/commit/9800b593034770fee5eb460ff0e8f0b13de83300 With this change, [in place variables](https://www.jetbrains.com/help/idea/exploring-http-syntax.html#in-place-variables) can be defined like this ```http @token={{$dotenv TOKEN}} ```