Improve [gigasecond]: ensure the analyser doesn't allow mutation of the argument

**Is your feature request related to a problem? Please describe.**
exercism/javascript#817 adds a new test requiring the `gigasecond` exercise function argument is not mutated.

**Describe the solution you'd like**
The `gigasecond` analyser should enforce that the function argument is not mutated (discussed [here](https://github.com/exercism/javascript/pull/817#issuecomment-571462819)).