Add a 'Current Location' button to the map

[aaronbrethorst]

User story

As a transit rider, I want to easily see my current location on the map, So that I can find nearby stops.

Requirements

• [x] add a 'Find Current Location' button to the map using the location-crosshairs icon. See StopPane.svelte for an example of how to use these.
• [x] Clicking the button should prompt the user for permission to their location and, if granted, re-center the map on the user's location and show a dot on the map indicating the user's location.

UI

The button should be located roughly where the purple box indicates.

[tarunsinghofficial]

I have done this task. PR is created.