Add styling for 'new' table row

A follow-up from gitlab!35708 (merged).

For gl-table, it would be nice to be able to designate a table row as a 'new' item in a list. We use this functionality already in the issues, merge request and (soon) alert lists, implemented in gitlab-org/gitlab.

Example:

This could be done either through adding a prop in the template, or by adding a utility class that provides the correct color.

The current color in use in gitlab-org/gitlab is $issues-today-bg;. This color is override-able, so it varies depending on whether dark mode is in use.