Option to generate pointers to fields on struct pointers

After we address https://github.com/dart-lang/sdk/issues/41237.

We can add an option to generate extension methods for `Pointer`s to fields of `Pointer`:

If we settle on an API like this for getting the offsets:

```dart
class MyStruct extends Struct {
@Int32()
external int foo;
@Int64()
external int bar;

@OffsetOf(#foo)
external const int offsetOfFoo;
@OffsetOf(#bar)
external const int offsetOfBar;
}
```

Then we could generate extension methods such as:

```dart
extension MyStructFields on Pointer {
Pointer foo => Pointer.fromAddress(address + MyStruct.offsetOfFoo);

Pointer bar => Pointer.fromAddress(address + MyStruct.offsetOfBar);
}
```

Which would be handy for cases like https://github.com/dart-lang/ffi/issues/117.