[Feat]: --mod-* tokens for `sp-tabs-overflow` shadow color

### Code of conduct

- [X] I agree to follow this project's code of conduct.

### Impacted component(s)

sp-tabs-overflow

### Description of the requested feature

sp-tabs-overflow doesn't have mod token to override the -sp-tabs-overflow-shadow-color , sp-tabs-overflow-next-button-left and sp-tabs-overflow-next-button-right for left and right button. Can it be added in the future? The reason is that I'm adding overflow tabs to the Properties Inspector, which has a background color of spectrum-gray-50, while the button shadow uses -spectrum-gray-100

### Mockups or screenshots


### Implementation notes or ideas

_No response_

Start with the sp-tabs-overflow component implementation, which is the entry point named in the issue, and trace its shadow color and next-button styling. Compare the existing token patterns there, then confirm that the left and right button shadows can be overridden for the Properties Inspector background without changing the default appearance.

Written by the indexing model from the issue text.