[meta] Missing type system features

Nobody has claimed this yet.

This issue attempts to collect and document missing or upcoming type system features that would be necessary to better type the public library interface, as well as mypy bugs preventing us to make use of existing features.

- [ ] **Function (and method) `**kwargs`**
To correctly and extensively type `**kwargs`, we would need at the very least the ability to tell type checkers that additional keyword arguments are allowed (and of type `Any`).
- **Current status**:
The codebase is currently typed with `**kwargs: Any`, but this disallows IDEs from discovering extra keyword arguments that are forwarded to other functions. For example, this is a very common pattern across the `Mobject` and `OpenGLMobject` inheritance trees.
- **Required features**:
- Allowing extra items in `TypedDict`: [PEP 728](https://peps.python.org/pep-0728/#allowing-extra-items-of-a-certain-type) has been [accepted](https://discuss.python.org/t/pep-728-typeddict-with-typed-extra-items/45443/159) but we need:
- support by mypy https://github.com/python/mypy/issues/18176
- pyright support to stabilize (it's currently marked as experimental) https://github.com/microsoft/pyright/issues/10803
- **Optional features** that would improve ergonomics/maintainability:
- in-line `TypedDict` definitions: there's a draft PEP for this ([PEP 764](https://peps.python.org/pep-0764/))
- **Notes**: encountered in #4398, discussion [on Discord](https://discord.com/channels/581738731934056449/1406957940329676830).

---
Related work:
- #3375

Start by reading the issue's references to PEP 728, mypy issue #18176, and pyright issue #10803, along with related issues #4398 and #3375. The issue is complete only when the required type-system support and its impact on the public library interface are defined and implemented, but it does not name project files or tests.

Written by the indexing model from the issue text.