Fix type inferencing and syntax highlighting for read/write property access (i.e. +=, ++, etc.)

Consider the following:
```groovy
class C {
  String foo
  void setFoo(String foo) { this.foo = foo }
}
@groovy.transform.CompileStatic
class D {
  void meth(C c) {
    c.foo += 'x'
    c.with {
      foo += 'y'
    }
  }
}
```

"c.foo += 'x'" infers as "setFoo" whereas "foo += 'y'" infers as "getFoo" (if it exists) or just property "foo".