Make TabsOverflow more extensible

### Code of conduct

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

### Description of issue

We have a use case in Hz where we would like to extend from Tabs and TabsOverflow. TabsOverflow currently [queries for sp-tabs in its slot](https://github.com/adobe/spectrum-web-components/blob/main/packages/tabs/src/TabsOverflow.ts#L66) and uses that as its scrollContent, but we would like to query for other elements to use as the scroll content. One idea is to create a protected getter for scrollContent so child classes can override it.

Start in packages/tabs/src/TabsOverflow.ts, especially the query for sp-tabs around line 66, and inspect how scrollContent is derived and used. Determine the smallest extensibility point that lets subclasses provide different scroll content; done means a child class can override that behavior without changing the existing TabsOverflow use case.

Written by the indexing model from the issue text.