TYP: define `subset` arg in `Styler`
This works but someone with more knowledge might be able to point out a better definition of the type for `Subset`,

 i.e. here `Subset = Union[slice, Sequence, Index]`

It types all the `subset` args in `Styler`
cc @simonjayhawkins if any comments thanks @attack68 

if you can create a followon issue to try to integrate this to the rest of pandas; e.g. move Subset to _typing and then *use it* in .dropna, and so on (e.g. where ever we use a subset argument), may need to add a todo list > This works but someone with more knowledge might be able to point out a better definition of the type for `Subset`,

would/could/should this be the same as PositionalIndexer in pandas._typing