Use `Sequence` type hints in `.transform_...()` methods of `TopLevelMixin`

[Antyos]

What happened?

I have noticed that several methods use list type hints instead of Sequence (which is covariant). For example:

altair/altair/vegalite/v6/api.py

Lines 2696 to 2701 in 58661c6

	def transform_aggregate(
	self,
	aggregate: Optional[list[AggregatedFieldDef]] = Undefined,
	groupby: Optional[list[str | FieldName]] = Undefined,
	**kwds: dict[str, Any] | str,
	) -> Self:

What would you like to happen instead?

All instances of list in type hints for various mixins should be replaced with Sequence (or potentially Collection, if applicable).

I think there is some level of codegen involved for the files in vegalite/v6/, so I'm not so sure where to start if I were to make a PR.

Which version of Altair are you using?

main branch