Add additional typing hints (#53)

* use labeled tuple elements for Point type

Using labeled tuple elements allows editors to show what values are expected in the different positions of the tuple.

* use mapped object type for Unit/Factors

Using a mapped object type allows the acceptable units names to be shared between the Factors type and the arguments to fromTile and the constructor.  This gives good editor support for the acceptable options to these methods.

* use labeled tuple elements for BBox type

Using labeled tuple elements allows editors to show what values are expected in the different positions of the tuple.

cheap-ruler.d.ts

@@ -1,25 +1,28 @@
-export type BBox = [number, number, number, number];
-export type Point = [number, number];
+export type BBox = [west: number, south: number, east: number, north: number];
+export type Point = [longitude: number, latitude: number];
 export type Line = Point[];
 export type Points = Point[];
 export type Polygon = Point[][];
 
-export interface Factors {
-    kilometers: number;
-    miles: number;
-    nauticalmiles: number;
-    meters: number;
-    metres: number;
-    yards: number;
-    feet: number;
-    inches: number;
+export type Unit =
+    'kilometers' |
+    'miles' |
+    'nauticalmiles' |
+    'meters' |
+    'metres' |
+    'yards' |
+    'feet' |
+    'inches'
+
+export type Factors = {
+    [name in Unit]: number;
 }
 
 export default class CheapRuler {
-    public static fromTile(y: number, z: number, units?: string): CheapRuler;
+    public static fromTile(y: number, z: number, units?: Unit): CheapRuler;
     public static units: Factors;
 
-    constructor(lat: number, units?: string);
+    constructor(lat: number, units?: Unit);
 
     public distance(a: Point, b: Point): number;
     public bearing(a: Point, b: Point): number;