To highlight all the missing annotations to fix and autofix `None` return

Excluding `*args` on `ChartType` wrappers. They need to be defined in alignment in multiple places, which is more complex

…`dict`

Among these, many locations already assume `str` keys in the implementation - without checking

```
altair\vegalite\v5\api.py:4368: error: Argument 1 of "__iadd__" is incompatible with "__add__" of supertype "TopLevelMixin"; supertype defines the argument type as "Chart | RepeatChart | ConcatChart | HConcatChart | VConcatChart | FacetChart | LayerChart"  [override]         def __iadd__(self, other: LayerChart | Chart) -> Self:                            ^~~~~~~~~~~~~~~~~~~~~~~~~ altair\vegalite\v5\api.py:4368: note: This violates the Liskov substitution principle altair\vegalite\v5\api.py:4368: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides altair\vegalite\v5\api.py:4376: error: Argument 1 of "__add__" is incompatible with supertype "TopLevelMixin"; supertype defines the argument type as "Chart | RepeatChart | ConcatChart | HConcatChart | VConcatChart | FacetChart | LayerChart"  [override]         def __add__(self, other: LayerChart | Chart) -> Self:                           ^~~~~~~~~~~~~~~~~~~~~~~~~ altair\vegalite\v5\api.py:4376: note: This violates the Liskov substitution principle altair\vegalite\v5\api.py:4376: note: See https://mypy.readthedocs.io/en/stable/common_issues.html#incompatible-overrides
```

This could later be extended to other modules, but for now `api` is complete.

To align with the other `ChartType`s