Less verbose Mock declarations

**Problem**
Currently defining a Mock looks like this:

```dart
class _MockDog extends Mock implements Dog {}
final Dog mockDog = _MockDog();
```

**Solution I would like**
I would like to have a one-line syntax alternative (or completely remove the old syntax):

```dart
final Dog mockDog = Mock();
```

The advantages are:
- Reduces the chances of creating a public Mock being shared, which leads to non self contained test files.
- Reduces file pollution, sometimes a test file can have multiple mocks and the class definitions of such pollutes the readability of the file.
- Somewhat forces developers that want to have any directly implemented
overridden fields or methods in a Mock to use a Fake. As detailed by the documentation (["A class which `extends Mock` should not have any directly implemented overridden fields or methods"](https://github.com/felangel/mocktail/blob/39d40c653819d09385af8955ef5d7259c58c9a58/packages/mocktail/lib/src/mocktail.dart#L79) ).

**Additional notes**
I'm willing to work on a pull request for this change if there is an agreement on the new syntax. As far as I am concerned having something exactly like the proposed solution would lead to a breaking change (depends on Dart's current and future capabilities). I'm opened to read about solutions that will avoid introducing a breaking change with the new syntax. I believe that a breaking change can be justified since the new syntax has many advantages and solves a couple of problems (as outlined in Additional Context).