[Leaflet] Making an L.Control class that extends L.Class so you can extend Control (#13954)

* Need to make static Class functions available on Control so you can extend Control class

* The latest leaflet 0.7.x added the PositionString type to the L namespace

* Specifying Control to only be a class that extends Class

* Synching back the mapzen changes

leaflet/index.d.ts

@@ -963,7 +963,7 @@ declare namespace L {
         position?: ControlPosition;
     }
 
-    export interface Control {
+    export class Control extends Class {
         getPosition(): ControlPosition;
         setPosition(position: ControlPosition): this;
         getContainer(): HTMLElement;

leaflet/leaflet-tests.ts

@@ -390,3 +390,10 @@ class MyDivIcon extends L.DivIcon {
 		super();
 	}
 }
+
+let myControlClass = L.Control.extend({});
+let myControl = new myControlClass();
+
+L.Control.include({});
+L.Control.mergeOptions({});
+L.Control.addInitHook(() => {});