Add optional check for whether a variable can be readonly

[Flu]

Adds an optional check for triggering a SC2331 for variables that are declared (and/or initialized) but never assigned to again. If a variable is found that matches, a note is given to maybe declare it as readonly.

Also implemented tests for it, as well as for the option itself in Checker.hs

In accordance to #2987, but it's not the full issue.

Let me know if you want to implement the full issue suggestion, as part of the same optional check, another optional check, etc.