Change access modifiers for Tabs methods to be more extensible

[lehelen19]

Code of conduct

• I agree to follow this project's code of conduct.

Description of issue

We would like to access more parent properties and methods in Tabs from child classes:

• https://github.com/adobe/spectrum-web-components/blob/main/packages/tabs/src/Tabs.ts#L303
• https://github.com/adobe/spectrum-web-components/blob/main/packages/tabs/src/Tabs.ts#L181
• https://github.com/adobe/spectrum-web-components/blob/main/packages/tabs/src/Tabs.ts#L164
• https://github.com/adobe/spectrum-web-components/blob/main/packages/tabs/src/TabsOverflow.ts#L104

[najikahalsema]

SWC-796