Should the type of `missing` be exposed publicly for use in type annotations?

Nobody has claimed this yet.

I almost filed a PR to make this change, but then I realized that the desirability of this is non-obvious.

I have some code which manipulates a variable like so in a pre-dump hook:
```python
class MySchema(Schema)
raw = fields.Raw()

@pre_dump
def do_pre_dump(self, obj: dict[str, Any], **kwargs: Any) -> dict[str, Any]:
try:
x = complex_func()
except ValueError:
x = missing

return {"raw": x}
```

When adding annotations, how do I annotate `x`? Assuming `complex_func()` returns a `T`, then

```python
x: _Missing | T = complex_func()
```

seems correct. But `_Missing` is an internal detail, not part of `marshmallow`'s exports.

It is not necessary for `x` to ever have a value of `missing` if you have another sentinel value to use (e.g. `None` works for many cases). I've refactored to handle this in my specific case, so there's no urgency here.

I have a branch which renames `_Missing` to `MissingType`, exposes it, and sets `_Missing = MissingType` as a bit of compatibility for anyone using it. But I'm not sure if I should submit the PR?

Start by reviewing the current definition and public exports of `_Missing`, then compare them with the proposed `MissingType` rename and compatibility alias described in the issue. The work is done when the public type-annotation API decision is resolved and any resulting change preserves the stated compatibility concern.

Written by the indexing model from the issue text.