interpreterbase: Add validator keyword argument to typed_kwargs

This attribute is a callable that returns a string if the value is
invalid, otherwise None. This intended for cases like the `install_*`
function's `install_mode` paramater, which is either an int or the
string "preserve", which allows us to do nice things like:

```python

class Kwargs(TypedDict):

    install_mode: T.Union[int, T.Literal['preserve']]

@typed_kwargs(
  'foo', KwargInfo('install_mode', ...,
  validator=lambda x: None if isinstance(x, int) or x == 'preserve' else 'must be the literal "preserve"),
)
def install_data(self, node, args, kwargs: 'Kwargs'):
  ...
```

In this case mypy *knows* that the string is preserve, as do we, and we
can simply do tests like:
```python
if kwargs['install_mode'] == 'preserve':
   ...
else:
   # this is an int
```

0 files changed, 0 insertions, 0 deletions