TabBar/Tab should only have one tab stop for accessibility

This is related to this issue: material-components/material-components-web#4429

On first render every tab in the tab-bar is a tab stop, since this is the default behavior for buttons without a tabIndex. When selecting another tab, the previously selected tab has its tabIndex state set to -1 meaning it is no longer possible to tab to.

A more accessible solution would be to ensure every tab except the active tab has tabIndex set to -1. This way there is only one tab stop for the tab-bar (assuming no one sets two or more tabs as active, but that is probably acceptable). Navigation between tabs would then occur with arrow keys as is already possible.

[moog16]

merged #691. Thanks for fixing and opening this issue 👍