Move dartdoc examples to separate files

As of recent `dartdoc` versions it's now possible to do:
```dart
/// My documentation string...
///
/// {@example /path/to/example/file.dart}
void someMethod() { ... }
```

Doing this makes it easy to run `dart analyze` on examples and use `dart format` on examples.

I propose that we refactor the current repository layout such that we have:
```dart
example/
flutter_webcrypto_demo/
... everything that's currently in example/
webcrypto/
hmac/
import_raw_key.dart
...
/
.dart
```

And then we use `/// {@example /example/webcrypto/hmac/import_raw_key.dart}` in our API docs.

This should not land as one large PR, please do:
* [x] One PR moving `example/` to `example/flutter_webcrypto_demo/`
* One commit that moves files
* One commit that fixes CI
* Any further cleanup of names, do that in an other PR
* [ ] A PR for each primitive, so we don't have too big PRs to review