Add transit line name popup on hover

A follow up from #658, add a popup with the transit line name when a user hovers over a line on desktop. Style of popups will be determined by line type, logic and styling to be re-used in neighborhood detail map:

Start by reviewing issue #658 and the existing desktop transit map interaction. Define the hover popup behavior for transit line names, including styling by line type, and structure the logic and styling for reuse in the neighborhood detail map. Done means the popup appears on desktop hover and the shared behavior works in both map contexts.

Written by the indexing model from the issue text.