Add option to prevent all kinds of top-level variable re-assignments

[leonklingele]

Currently, only re-assignments to top-level variables of another package are detected. It would be nice to also prevent re-assigning any top-level variable so they are read-only.

[chokoswitch]

Thanks @leonklingele - this option was originally left out because it can lead to more false positives than overriding imports and that actually the code for handling it would look very different, not using the import mechanism. There are some things to think about, for example presumably the assignment should be allowed within an init() function as it's a common pattern.

As an explicit opt-in pattern though, it may still be useful, will look into how it could be added to the linting code