Proposal: add text.selection tokens

From @clavimoniere in Slack:

My proposal is that we leave this in place for now and in future add some sort of token like text.selection.background.default and set it to blue 100. that token can have some siblings and cousins later if necessary, like text.selection.color.default if we want to theme the color of the selected text, text.selection.background.strong if we need something else for selection on a strong background, etc.

Related to gitlab-org/gitlab#472881