Make a rule for variables declared/assigned without an explicit type

So all of these variable declarations are valid in groovy:

```groovy
def foo = 'bar'  // Bad, but CodeNarc will catch this with NoDef
String foo = 'bar'  // Ideal
foo = 'bar'  // Bad, but CodeNarc won't catch this
```

That last case should be treated as a violation. It is technically an assignment, but lacks any explicit type.