Support dashes in variable names

Using variables in headers, triggers the request passing `${[ the-variable-name ]}` as header value.

Note that renaming the variable name from the-variable-name to the_variable_name it started to work.

According to the documentation, there is no mention to naming convention or restrictions to environment-variables:

https://feedback.yaak.app/en/help/articles/3284139-environments-and-variables

Is this a bug, or a undocumented feature? :)
I would rather not limit the possibilities of naming the variables, and allow dashes in them too.