Implement <map-feature> custom element

[prushforth]

We should try to implement <map-feature> as a custom element. Some behaviour that this should encompass, tentative list:

• ability to add/remove <map-feature> as a child (or perhaps descendant) of <layer-> and render / remove the rendered resulting markup (SVG).
• perhaps: modify the coordinates and have those modifications rendered dynamically
• provide internal support for returning a GeoJSON representation of <map-feature> geometry. The objective is to be able to use turf.js to sort and analyze features for accessibility support e.g. sort ascending on distance from map centre
• per discussion with @AliyanH, a custom element could directly provide interactivity, such as a .click() method, so that applications could work with features on the map without having to delve into our shadow DOM
• method to calculate and return the extent of the feature as a JavaScript object