[Feature Request]: linter rule for unused variables, generally

[jabraham17]

We currently have several different "Unused*" rules in chplcheck.

• UnusedFormal
• UnusedLoopIndex
• UnusedTupleUnpack
• UnusedTaskIntent
• UnusedTypeQuery

All of these rules have very similar logic to compute them, and could probably be refactored to a common rule. Even if we want to keep them as separate rules (although I don't know that there's much utility to that), they can at least share common code.

From that mindset, these all have the same theme: "unused var-like thing". However, we don't have a lint rule for just variables, and I think this should be added.

We could also consider UnusedFunction and UnusedType (records, classes, enums, etc), although this may be more annoying than anything else. Especially as we start to consider Chapel libraries, the place where the function is used may not be available. One limited possibly here is to only check for UnusedFunction/UnusedType inside a limited scope like a function body.