Add option to disable variable_separator in RenameVariables transformer.

[fialsha]

Enhancement request: add option to disable variable_separator in RenameVariables transformer.

Currently, RenameVariables:variable_separator can only be configured to underscore or space.
We would like to disable it completely, but still use rest of the capabilities of RenameVariables transformer.
Something like:

robotidy -c RenameVariables:variable_separator=False

Use case
We use variables in the following format to represent nested values in YAML files:

MY_VAR__NESTED_VAR_1__NESTED_VAR_2

The RenameVariables:variable_separator transformer interferes with these variables.

[bhirsz]

I've looked in the source code and it looks doable. We always convert to one format and then either to space or underscore but I can add option to not convert at all. Since the parameter is str (underscore / space) I would add third value (ie "ignore")