Support @ and : in variable masking

Problem to Solve

As part of https://gitlab.com/gitlab-org/gitlab-ce/issues/60790 we want to include the @ and :.

We're missing a good test plan for testing the behavior of masking these characters across different executors (shell, ssh, docker, ...) on different platforms (Windows and Linux). I feel this is a prerequisite before we can move forward with adding the characters, because we need to make sure it won't cause any breakage.

Limitations

The special character \ will remain unsupported for now due to ambiguity in escape sequences making it challenging to implement.