Tyro

Re-add the `py.typed` marker, which was inadvertently deleted.

- Documentation refactor.
- Serialization utilities have been moved to `dcargs.extras.*`.

- Supporting using built-ins directly: `dcargs.cli(int)`, `dcargs.cli(List[str])`, etc.

- Support for nesting collections. (tuples inside tuples inside dictionaries, etc)
- Overhaul mechanisms for handling `Union` types, which are now supported recursively and much more generally (eg `Union[int, str]`, `Union[Tuple[int, int], Tuple[str, str]]`, etc).
- Support `total=False` for `TypedDict` types.
- Helptext generation improvements: conciseness, ignoring ANSI codes when wrapping.
- Add `prog=` argument to `dcargs.cli()`.
- Various minor stability improvements.

Niche improvements for nesting:
- Optionals:
- Optional types in containers (eg `List[Optional[T]]`).
- Optional nested structures (eg `Optional[SomeDataclassType]`).
- Remove limit on of nested unions/subparsers (previously: one union over child structures per parent class).

Enabling more useful "base configuration" design patterns via `default_instance`:
- Introduce `dcargs.MISSING` for marking arguments as missing defaults.
- Introduce `avoid_subparsers` argument for `dcargs.cli()`.

- Added support for general dictionary annotations, via `typing.Dict[K, V]`.
- Refactored argument definition backend, which fixes some helptext corner cases for nested positional arguments.