Add type hints.

[fabiocaccamo]

[pirate]

It would be great to be able to define the key type and value type generically like Dict[str, str], e.g.:

from benedict import benedict
from pydantic import TypeAdapter  # just used for deep-checking equivalent to isinstance()

def example_function() -> benedict[str, int]:
    return benedict({'abc': 123})
    
TypeAdapter(benedict[str, int]).validate_python(example_function())
# this should succeed

But also separately I think it would be cool to be able to use them like TypedDicts as well, with hardcoded (total=True/False keys) that are always expected to be present, e.g.:

from benedict import TypedBenedict

class MyCustomBenedict(TypedBenedict, total=True):
    abc: int
    xyz: str
    
test1 = MyCustomBenedict({'abc': 123, 'xyz': 'test string'})
# this should work

test2 = MyCustomBenedict({'abc': 123, 'notallowed': 'something else'})
# this should raise an error because notallowed is not present in the spec