In some areas of the world, ship transportation is the main means of long-distance travel. For example, in Alaska, the [Alaska Marine Highway System](https://dot.alaska.gov/amhs/route.shtml) is the principal means of transportation through the Alaska Panhandle and down the Aleutian island chain. The AMHS carries the same importance that the Interstate Highway System has on land, and these ferry routes are exceptionally long distance.
This PR addresses that by:
* rendering oneway arrows on more than `subclass=cycleway`
* less opacity for path/footway casing
* adding the thin black casing to bridges
* adding `subclass=steps` & `subclass=bridleway` rendering
* render `bicycle=designated` as `subclass=cycleway` and `horse=designated` as `subclass=bridleway` (bike taking priority)
* improved tunnel rendering with double casing & adding main line
* made casing min zoom consistent across subclasses
All of these changes were made to mimic OSM Carto.
* OpenMapTiles style
* Rename style snippets from layer_name.json to defaultstyle.json
* Add README to icons dir.
* Add README to icons dir.
* Rename snippets to style.json
* Fix spritezero - move icons to svg subdir
* Move icons from style/icons/svg directly to style/icons
Co-authored-by: Tomas Pohanka <TomPohys@gmail.com>
