location field: override icons (possibly using Slots pattern)

[fpurcell]

Our components will eventually need to allow the caller to override icons and other styling.

[fpurcell]

Beyond using different sets of icons from the defaults used in otp-ui, there is a secondary need to broaden the icon pallet based on the 'layer' type of a given geocode record (e.g., from the list of records returned by the geocoder, 'addresses' type records will have a different icon from 'stop' records, which may be different from 'venues' / 'landmarks', etc...).

In the example below, the two stop records would get this icon:

...whereas, the other layer types would get another icon. (see #13 , which talks about returning more data from the geocoder, and #14 which talks about overriding the icons).