This PR is an alternate version of
https://github.com/zed-industries/zed/pull/22850, but now using a
similar approach to the existing `tab_content` and `tab_content_text`,
where `tab_tooltip_content` refers to the existing `tab_tooltip_text` if
there's no custom tooltip content/trait defined, meaning it will
simplify render the text/string content in this case.

This is all motivated by
https://github.com/zed-industries/zed/pull/21955, as we want to pull off
the ability to add custom content to a terminal tab tooltip.

Release Notes:

- N/A